I prefer the 20" barrel AR15 while out-of-doors. For indoor work, I prefer the carbine. I spent a heck of a lot money having a custom built 20" upper which is for long distance work where accuracy is more critical. I spent much less for the Bushmaster carbine where accuracy is less critical for my purposes.
The 20" rifle has free-floated custom barrel and a Wilson TTU 2-stage trigger.