A few questions as to the pros/cons of each.
1. Accuracy - that much difference?
2. Compactness - in vehicles, urban environs, etc. - not concerned with bayo usage
3. Free float - what free floats can be used with a permanently attached FH?
4. Twist - 1:7 or 1:9 - faster twist won't stabilize (as well) in shorter barrel?
5. Overall benefits to one or the other?
The difference in length between a 14.5 with a permanent flash suppressor and a 16" weapon with an A2 suppressor is very, very small. I played around with 14.5s with permanent suppressors, but it got expensive and I couldn't swap out my rails and such. I prefer 14.5 as a barrel length, but get 16" barrels now because I cannot SBR them in my state.
You will not notice a difference in accuracy. At a distance the wind will push the 14.5s bullets a bit more, but if you are a good enough shot to notice the difference you can probably dope the wind well enough it doesn't matter. Compactness.....is pretty much a draw. The difference is not very noticeable. If you put the rail on BEFORE you attach the suppressor, you can use anything. Otherwise look into the SIR and the RAS 2. There are more options than that, but I don't know much about them. For twist...if you are going to shoot ONLY 55gr and 62gr ammunition, 1/9 will serve you well. If you want to use 75 or 77gr ammunition later, get 1/7. There is no downside to 1/7, but 1/9 is limited. I would say just get the 1/7.
All in all, I would say you are best off with a 16" 1/7 barrel, preferably a mid length. 14.5 is nice on an NFA weapon, but it is not worth the extra work on a title 1 (non-NFA) weapon.
I have decided for my stable of rifles the following:
14.5" builds, carbine length gas, perm flash suppresor, 2 piece rail preffered (Troy, Samson, MI) but I've been using LaRue on most builds so if I do use LaRue I don't plan on messing around on changing rails. The trouble is you have to assemble with the one piece rail and then perm attach the FH.
16" builds will be mid length gas system. No worries about muzzle device or swapping rails.
I preffer the function of the 16" mid length. Remember a 16" with A2 FS is only aobut an inch longer then a 14.5 + phantom or vortex.
I will have a couple of 14.5's in the stable one M4 govt profile and one SOCOM profile. The rest of the carbine builds will be 16" mids. I have a Denny's operator build now and plan on a BCM gov't contour mid as soon as they are available.
1-7 twist is the way to go if you plan on shooting the 70+ grainers. I shoot mostly XM193 through my 1-7's with no issues. I don't think you would notice any accuracy difference.
I'd say 16" mid length gas, 1-7 twist, chrome lined
These are available from Denny's, BCM and Sabre.
What he said.
I have 14.5" 1/7 but thats because I have another AR15 with 20".
If I would have to choose one it would most certainly be 16" 1/7.