If the primary use were indoors, I would say just get the 12.5”. That’s what I got for my first AR...using the original SB Tactical arm brace. It’s since been SBR’d.
But practically speaking, the 16” avoids a lot of inconvenience and keeps you off of any lists. I’d probably grab the Colt CCU if I were planning to step out of my house and not get right into a vehicle. Get a 12.5 or 11.5” later on if you want to suppress.