I have 4 and 8 inch barrels for my MPX.  It spends 95% of the time with a large/heavy 45cal silencer(a bit over 8 inches long) on the 4 inch barrel because the gun is awkwardly long and front heavy with the big silencer on the 8 inch barrel.
So I'm getting a proper 9mm silencer for it.  

Both barrels shoot accurately.  I like the 8 inch barrel because it is handier unsuppressed and I have the 8 inch handguard.

Curious if anybody has input regarding a long barrel/short silencer combo vs. a short barrel/long silencer combo?  

I'd probably say you need to decide if you want it suppressed full time or not to help decide.  I have an MPX that came factory 8inch barrel but changed it to a 4.5in barrel planning to keep a dedicated suppressor on it full time.  I kept the longer handgaurd on it with suppressor tucked under.  

If I wasn't doing suppressed I'd keep it as 8inch barrel as I liked the longer handguard and it was still fairly compact overall

That’s the problem….  I like the 8 inch barrel and the 4 inch is too short without a silencer.  I also don’t want to be swapping out the barrels.  
Long barrel and new silencer is only an inch longer than what I have now so I’m leaning towards that.  

I was mostly concerned about the possibility of the 8 inch barrel being louder.  I’m feeling that is a longshot though as I can’t tell the difference between the barrels now.  

I bought a 6.5" version of the mpx. Don't have very much time on it. Looks pretty good with an Obsidian 9 in short config with factory 8" rail.
Probably would need a 10.5" MI rail to look right in long config.

Long config


Short

As long as it’s the same accuracy, I think I’d prefer the 4.5 w/ the suppressor tucked underneath the 8” hand guard.  

I specifically wanted an 8” mpx because I hate the look and feel of the 4.5” hand guard.

Mine is an 8” barrel and I only shoot it suppressed.  It looks good, but I think the best looking combination is the one mentioned above.  8” hand guard w/ a tucked suppressor.

Sorry to hijack your thread but.....

These have a left hand thread muzzle device from the factory right?  I'm looking to put a suppressor on mine and trying to figure out what mounting system I want.

Factory barrels yes.  If keeping the factory barrel some of the suppressor  companies make hubs/mounts with different thread pitches including the left hand pitch on these.

I swapped barrels on mine to a 4.5in ILWT which has normal 1/2inch threads.

But beware - all 13.5x1 mounts are not the same.  
Some shoulder on the taper, some require a taper adaptor to make a perpendicular shoulder on the taper, and some shoulder on the muzzle of the barrel.  
I would recommend you avoid the taper adaptor, I've never seen good results with those on the 13.5x1 threads.

The threads are designed to shoulder on the muzzle because they were designed for semi-auto pistols where the barrel was very close to 13.5mm OD.  
Additionally, the cylindrical surface ahead of the threads is there to provide a smooth surface for an o-ring to seal the mount to the barrel.  

Ecco machine makes a silencerco alpha mount that shoulders on the muzzle.  If you need an alpha style mount I would recommend it.

4.5” bbl with 1/2x28 and 3 lug adapter.
Yhm r9 w/tri lug adapter

I had my barrel cut down to 6" and run a dead air Wolfman with it.