I'm going with the 11.5" with an AAC M4-2000 with the brake style QD to be a little more gentle with the supressor. Approximately what would be the difference in velocity with 12.5 vs. 11.5?
Depending on the load, probably around 70fps or so. Also, with the longer dwell time of a 12.5" vs an 11.5" it should be more reliable. You would probably not notice any reliability issues until you get it really dirty (which suppressors tend to do).