10.5 because you can attach a silencer to it and have an OAL less than a 16" rifle with a birdcage.
John Noveske mentioned to Stickman once that if he could only have 1 AR, it would be a 12.5 since it is a good combination of velocity, length, accuracy, and I tend to agree with him.
But I'd imagine an 11.5" rifle of equal pedigree would be able to do the same in the hands of an equally trained shooter. Even Bill Alexander of Alexander Arms has used a 10.5" in 6.5 Grendel and successfully downed Larue Sniper targets at 600y.
Since you have the 11.5, look at doing a MK18 of some sort for a future project.
I have 2 10.5" uppers and my 7.5" 300BLK and I'm going to do another shorty but probably in 11.5 since I just bought a Centurion Arms 10" C4 from a friend of mine. It may become a 10.5 if I find my suppressor mount clears it. The 11.5 I have brewing in my mind may end up being made with a URX4 10.75 rail.