Collect them all!
I don't have a carbine as both my ARs are midlengths. My stealth is a 16/13.2 and my BCM wears a dd omega (didn't wanna pull the FSB off else it would have a larue 9.0). I feel that, for running and gunning, the 9 is fine. A 10 would be slightly better for more hand placement options, but remember that I have the FSB to contend with on that stick. I would stop there on a carbine... I love my stealth's 16/13.2, but that's my precision rig and it isn't getting the same type of use my BCM gets. When I've got a few more sheckles, I'm throwing a LaRue/Harris bipod on it at the very end, which I think will make it the most ideal rail length... for that type of gun.