I am about to "rearrange" my AR collection and have been pondering the same Q's.
I currently have 16" 5.56 and 6.8 SPC II, 10.5" 5.56, 11.5" 6.8 and 9.5" 300BLK for my 17yr old son and I.
The 10.5" 5.56 with a Fortis rail and 11.3oz, 6" long DT and flat end cap Omega is my favorite set up. Light, short, handy. My 11.5" 6.8 is second favorite, an YHM ULT suppressor is dedicated to it. I love the 6.8's for hog hunting, but the 5.56 works just fine, so the 16" 6.8 is going. No reason to mess with the 16" 5.56 with Fortis rail; its reasonably light and handy and not unbearably long or heavy with an Omega so it is staying. I like playing with the 300BLK but ammo is ridiculously priced, especially sub sonic ammo, and even reloading doesn't bring cost down to bulk 5.56/223 ammo costs. It is a surplus toy that is going (after I shoot up the ammo I already have.)
So, here are my takes:
Gotta be 5.56/223 for ammo costs.
For instate only travel than an SBR. However, if last minute interstate travel is likely and time to file a Form 5320.20 limited then 14.5" brazed or pin and weld - but you need to know what your suppressor is going to be.
I am going to do both a 14.5" P&W and a 10.5" or 11.5" SBR upper. I know what suppressor is going on the 14.5", so I know what FH to P&W.
Faxon has a 14.5" very lightweight GUNNER barrel. It will replace the 16" 6.8, and with the 13" KMR hand guard currently on the 16" 6.8 it will make for a nice, lightweight non-SBR set up.
For the 10.5" or 11.5" upper, I am thinking either a Faxon 10.5" or lightweight 11.5" GUNNER barrel and a 9" or 10" KMR Alpha. I'll use a SilencerCo ASR FH and one of my Omega suppressors. This should also be a nice lightweight set up.
If HPA passes then I am thinking P&W a Spartan 3 suppressor to the 10.5" or 11.5" for a no stamp rifle. The Spartan is not too expensive, light and not too long, suppresses well and comes drilled for a pin. Takes a 10.5" to just beyond 16". That would free up an SBR lower and an Omega too.
Hope this helps!
JPK