TL;DR: See Bottom Line

I went through a fair bit of trouble making what I feel is the "right choice" for my BUIS. Supporting me throughout have been helpful reviews & discussions by members of this forum and elsewhere. This is my attempt to give back a little bit and hopefully help someone else out there who's struggling with choosing between these two.

First part's objective, latter half is my own experience. Formatting options are limited so I'm including pics at the end.


KAC Micro BUIS

Front
Weight*: 1.5 oz (source)
Stowed length**: 50 mm
Stowed height above rail: ~9.5 mm throughout; 10 mm @ knob
Material: Steel base, aluminum flip-up

*Preferred to use second-hand measurements over manufacturers' specs
**All dimensional measurements are my own

Rear (300 M version)
Weight: 1.35 oz (source)
Stowed length: 55 mm
Stowed height above rail: 10 mm measured @ tallest point (the sight base); 13 mm @ knob
Material: Steel

What's in the package:

Each sight comes sealed in a plastic baggie, in a cardboard box, wrapped in a cardboard sleeve. Package features product name on a label and the KAC address & logo on the sleeve.

Features & function:

Both sights flip up & down manually. Neither lock in the up or down positions. Both offer tool-less adjustment via knobs that can be manipulated by hand. The front is adjustable for elevation, the rear is adjustable for windage. The rear sight features a removable plastic insert to reveal a larger aperture. The front sight has a non-standard front sight post so it's not interchangeable with typical aftermarket posts, as far as I understand. The front sight post is also thinner than your standard USGI width (see comparison photos below).


Magpul MBUS Pro BUIS

Front
Weight: 1.52 oz (source)
Stowed length: 40 mm
Stowed height above rail: 9 mm throughout; ~11 mm @ knob
Material: Case hardened steel finished with Melonite

Rear
Weight: 1.79 oz (source)
Stowed length: 52 mm
Stowed height above rail: ~9.5 mm throughout; 10 mm @ knob
Material: Case hardened steel finished with Melonite

What's in the package:
Each sight comes bolted onto a faux picatinny rail insert made of plastic. Package includes detailed instructions with photos on how to install and a rough guide on the adjustment increments. The package itself details out product specs, etc.

Features & function:

Both sights flip up & down manually. Neither lock in the up or down positions. Both offer tool-less adjustment via knobs that can be manipulated by hand. The front is adjustable for elevation, the rear is adjustable for windage. This front sight also has a non-standard front sight post so it's not interchangeable with typical aftermarket posts, either. The front sight post appears to be standard USGI width (see comparison photos below). The rear sight features a flip-down peep insert that reveals the larger aperture, held in place by a detent. The rear sight surface surrounding the aperture is serrated with the intention of reducing glare.


Experiences - KAC

A lot of fanfare out there for the KAC Micros, so I picked up a set from Amazon for roughly $230. Their construction is fantastic - I completely disassembled the front Micro at one point and was pretty impressed with the attention to detail and quality of each individual component. The front sight is very easy to flip up and adjust. Best of all, the front sight picture is awesome - very thin and precise (pic below). Some out there claim that a thin sight means it takes longer to acquire than the standard USGI width in fast-paced situations, but I don't know - I've never been in a situation like that. I also think the design of the sight itself is pretty elegant - I know it's dumb but for some reason I find it more aesthetically pleasing than the MBUS Pro. The knob for elevation adjustment is easy to turn with positive clicks providing feedback. As for the KAC rear, it feels totally solid in the up and down positions. No play in any direction whatsoever. The windage adjustment knob provides for an easy grasp - but difficult enough to turn to prevent any accidental changes. Probably helps with gloved shooting. The removable rear peep insert is silly, though. I think that if you're anything short of OCD, you will lose this thing shortly after you take it out. Replacements are available on KAC's website for $2.38. There's a YouTube video of the owner discussing design proposals for a rear aperture and that they settled on this design, but unfortunately nobody was around to ask "why?" Maybe cost? Who knows. Aside from this minor nuisance, I think that the design philosophy of the KAC Micros is legit.

However, I did run into a couple of issues with the front KAC Micro. The first was that the front sight (part that flips up) exhibits some front-to-rear and side-to-side play within its own base (I installed it properly, etc.). I had not even noticed it until a few guys at my local range pointed it out. At first I wasn't really concerned since I thought it was minor and totally irrelevant, but after dwelling on it for a bit I spent some time figuring out whether my eye could discern the side movement (front/back play is less important IMO since the detent keeps the "up" position consistent). Well, turns out it could, and this made me uneasy since I wasn't confident it'd remain in a consistent position under recoil. I measured the side-to-side play with a feeler gauge @ 0.229 mm. It might seem insignificant, and maybe it is (basic math = ~2" of potential deviation @ 100 yards), but I don't want to introduce any additional error beyond the shooter. Some members on here claim that their KAC Micros are rock solid, so I ordered a replacement. The replacement has even more side-to-side play and also rubs the adjacent rail tooth over which the sight is supposed to hover, polishing my rifle's rail and making it more difficult to raise & lower. I called KAC and a very nice rep mentioned that he could replace them but could not guarantee that the replacements wouldn't have the same "issue." At this point I felt that I wasn't getting the quality that I expected, so I ordered a set of Magpul MBUS Pros to see what they were like.


Experiences - Magpul

I got the MBUS Pros from Amazon as a set for ~$136. I wasn't expecting much, so when I opened them I was totally blown away by how rock solid the entire assembly felt on both the front and rear sights. Absolutely zero play in any components in any direction. The finish on the MBUS Pros is also superior to the Micros. The bases of my own KACs and the ones I've seen on the internet appear to have a worn, delicate coating. So in terms of fit & finish, I can't see how the Micros "beat" the Pros as many people on the internet vehemently claim.

The front sight post is thicker and so are the legs, so I think I still prefer the front sight picture of the KAC. My groupings are better with the MBUS Pros for some reason, though - probably due to my preference for the rear sight's thicker surrounding around the aperture, which makes it easier for my eye to perceive when focusing (see pics below for comparison). The KAC rear unfortunately shows up as not much more than a thin ring when focusing at a distance which makes it tough for me to center (might be different for you). I also noticed that when the front MBUS Pro is stowed, there's a *barely perceptible* gap above the rail which keeps the sight from slamming into it - thought that was a nice touch. One thing to note is that the front's knob is quite hard to turn - I don't see this as a problem but I saw a few people mentioning they couldn't turn theirs at all without using some kind of tool. You may notice in the pics that the "Up" adjustment arrows on the front KACs and Magpuls are opposite - this is because in the Magpul's, it's the knob that's threaded and the front sight post that's "floating." On the KACs, the knob is what's "floating." (To envision this, go grab a bolt + nut and you'll see what I mean.) Another notable aspect is that that the Magpul front takes up significantly less rail estate than the KACs with no drawbacks (see pics). It also offers an extra "tooth" that mounts into rail in addition to the screw itself.

Switching between apertures on the rear MBUS Pro is far simpler than the KAC. I do, however, like the windage knob on the KAC better since it's chunkier, making it easier to grasp. I think Magpul might've been attempting to make their rear as low-profile as reasonably possible, thus the smaller knob which is more difficult to turn. One random note - the KACs are 84 clicks of windage end-to-end, while the Magpuls are 124. I'm not sure if this yields any practical advantage.

One final note - I do respect the effort that Magpul put into the MBUS Pros' packaging and whatnot. Some might call it marketing, but to me it's attention to detail and customer orientation.


The Bottom Line

I am willing to spend money to make my one-and-only rifle as good as it can be. Yet, I'm not sure what one gets for the $100+ extra to buy the KACs. I honestly liked them and wanted to keep them. But my first-hand experience revealed that the Magpuls offer as much as the KACs in a smaller package.  They also feel beefier with a more robust lockup and feature a more practical rear peep insert. I have absolute confidence that they'll maintain their composure under all types of shooting. If you have a solid set of KACs that you use and like, then that's awesome and nobody can fault you. For myself, I couldn't justify it after the issues I encountered and consequently discovering what the Magpuls offer.

Edit: My girlfriend wants credit for providing her fancy yoga mat for the pics. So thanks, girlfriend.

Bravo!

Nice review.

I've been running the KAC BUIS exclusively for some time now, but I will say that I wouldn't be opposed to putting the MBUS PRO sights on any of my rifles.  I've looked at them and they are a solid product.  That said, I prefer the KAC P/N#: 99051 front sight (not the micro) above all others.  Any reason you haven't considered the Spike's Tactical micros?

I just ordered my first set of MBUS Pro sights yesterday, I have several sets of Troy Ind. sights and a few others, makes me want to do a similar comparison, its to bad the MBUS Pro dont have the option for tritium posts though

Honestly, no reason besides that I'm new to this game and KAC and Magpul were the designs that seemed the most "seasoned" when I started looking. The Spikes ones look promising, too - I just never expect to have to shoot far enough with irons to need a rear BUIS w/ elevation. The peep removal thing also seems like it'd be annoying, but I like the detail that they put into the different materials to make them as light as possible.

I'd put those next on my list to try if I had to give up my MBUS Pros. Also, at the price point of the Magpuls & Spikes, I think KAC should be concerned.

Edit: After looking at the Spikes a bit more I think they might just have a great product with not-so-great marketing... Their own sights are last in a long list of Magpuls on their own website?? Don't get it.

Very nice side by side photos, and MBUS PRO!

I'm finding that I really like these... The fatter front post makes it easier to pick up for me.

Mbus pro are a great value for the money. I have 2 sets.