I guess I'm somewhat alone in this -- I suggest getting the 14.5" mid length first and a 11.5" upper receiver later (if you decide you want a second one later).
Mid length means lower chamber pressures, making it easier on the extractor and increasing the lifespan of all the major rifle components. It also makes for a smoother shooting carbine. 14.5" means the overall length isn't too much but still has more velocity than the SBR. I have a BCM BFH 14.5" ELW KMR-13A upper receiver with a pinned SureFire SFMB556. It provides a lot of space for mounting my night vision equipment while still leaving space for comfortable handling. The thing shoots flat and still handles well when suppressed.
If you get a 14.5" upper receiver with a pinned muzzle device and buy a second lower, you have a rifle that can be taken to any of the non-communist states legally on a short notice without filing a Form 20.
The 11.5" is nice and handy, but shoots rougher and gets really concussive with brakes or some compensators. It's nice especially if you care about overall length when you want a suppressor later.
I really like SBRs, as I have two AR SBRs, two SCAR SBRs, and an AK SBR, but I still find the 14.5" mid length lightweight a very good balance of benefits between length, weight and shootability. Whenever I leave the house, it's the first thing I grab.