Go there.

There is also a separate "Group buy" sub-forum somewhere around here.  SKD always seems to be running some sort of group buy.  

Check SWFA also. Last summer I got a great deal on their "sample list" of barely used demo optics.  <$400 with QRPII mount, ARD, and flip caps.

Best of luck.    (also, recommend just buying an H-1, T-1, cuz if you are anything like me, you will want to kick yourself for not just getting it the first time)

Weight is the primary reasons.   Any mass above the barrel really screws with the balance of a rifle in my opinion.  A mounted Aimpoint micro feels significantly lighter and faster than an M2/3/4.  I would not say that the CompM2 is heavy at all, but every bit counts as you start adding equipment onto a rifle.  In reality, I probably will not be upgrading to a micro for a while.  The CompM2 really does a great job.  There is an advantage to the large objective of the CompM2, but I prefer an absolute co-witness with my iron sights, so I don't think it is as much of a benefit as if I were using a 1/3 lower co-witness.  Newer aimponts retain value quite well, so if you want to upgrade later, you can do so at little loss.

If you are not familiar with the co-witness terms, you can look it up with a search for good descriptions, but I'll give you the basics.

First an understanding of the RDS:

The red dot in the aimpoint does not have to be centered to hit the target.  The red dot will hit whatever it covers within certain ranges no matter if it appears to be in the center, corner, top, or bottom of the objective.

Secondly.  Shoot with both eyes open.  Look past the scope, and at the target.  Red dots function correctly even if the front lens cover is down. Fuzzy dot on badguy? Like magic, it will hit!  These are all about speed.

Now a quickie explanation of absolute and lower co-witness.  This refers to the relationship between your iron sights and the objective lens of your optic.  If you line up your front and rear iron sight, as if you were shooting irons, and the dot is centered in the objective right above your front sight post, you have an absolute cowitness.  Lower 1/3 is when your iron sights line up in the lower 1/3 of the the objective.  Again, the red dot will track down to your front sight post.(magic!)  This gives you a little more "open" space to see the target as your sights will not occlude anything.  I use absolute cowitness because I think if the dot itself is ever overpowered by light (sunlight/flashlight reflection) your irons will immediately sihlohette(sp?) and give you a sight picture to shoot.  There are arguments for and against either position, and I am not sure if I know enough to make a decisive decision either, but it works for me right now, so I will stick with it until someone with a lot of experience gives me reason otherwise.

Good luck!