I imagine the UBR is the best choice but it's pricey.  I am leaning towards the ACL or a fixed A2 or MOE.  Is the ability to change LOP in positions more valuable than having that smooth rifle length buffer in rapid fire?  I have been out of the game for some time but the kids are old enough for me to start chasing a distinguished badge that I should have chased when I was younger.

I don't really adjust my stock for different positions, but then again, I don't shoot competitively yet either. I like having a collapsing stock because I run my stock normally shorter than the A1/A2 length, and when I'm loaning it out to friends, they can put it where they like it. My current favorite from the ones I've handled, and what I'm running right now, is the VLTOR IMOD Clubfoot. If I went to a fixed stock for whatever reason, I'd likely either run the VLTOR option, or the Ace ARFX.

As for recoil impulse, I'd say that you're probably better off just getting a mid length system with an adjustable gas block than a rifle length buffer and fixed stock. Mid vs Carbine length gas is noticeable for sure, but collapsible vs fixed (both on the same middy) wasn't anything I can remember noticing.

20" barrel for service rifle, so rifle length.  Rifle length gas, rifle buffer, 12+ pound rifle and an A2 compensator make for a pretty flat shooter in rapid fire and they hang nice in offhand.

Oh, yeah, we're running totally different rifles, I'm at 13.8" pinned for HD and hog. For service rifle, a fixed stock would probably be nice if you don't have any issues with the length. Consistency in position and cheek weld is never a bad thing, and since handling isn't a concern, it's not like you'll want to scrunch it down to get it in and out of cars. Might as well go for the comfort of an A2.

Guess I'm just used to everyone running Mk18 clones and M4gerys.

Most UBR users go 2 clicks in for Offhand, then use the prone setting for Sitting.

I'm slowly building a "modern" SR, and will use a snug waffle stock with some padding on the exposed tube.

ETA: aren't there adjustable gas keys to help reduce recoil? That would be legal instead of an adj. gas block.

I have a D stock(A1) on my A2 (20" govt)