I've shot just about every variation out there extensively and it's hard to beat a 10.5-11.5" for all around usefulness and fun.
7.5"s are neat and tiny, but vicious indoors, unless suppressed. 10.5-11.5"s with a good flash hider (i.e. AAC Blackout) aren't noticeably louder than a 16" carbine and can be suppressed without being cumbersome. 14.5"s are nicely balanced (without a suppressor,) but don't provide much over a 16", IMO.
I don't get any enjoyment out of the concussive fireball at the end of my barrel, so the above was written from that standpoint.
[edit] Kinda missed the point, didn't I? The obvious choice is to SBR an AR-15. It opens up a whole realm of options for easy upper swaps, whether that be to change barrel length, chambering... whatever.