This is really about personal preference and your use case for the gun.
Personally… for shooting without a silencer I would go with a 12.5" to 16" barrel. For shooting with a silencer I would go with 11.5"
If you only plan on shooting out to 100 yards then 10.5" going to work out just fine. If you want to extend that out into the 200 to 300 yard range then that’s where you start to gain advantages with 11.5" or 12.5". If you want to shoot at something 500+ yards out then that is where your 14.5" to 16" barrels come in.
People really love their MK18 clones, but are they doing CQB training in a shoot-house with that thing? Probably not. Those rifles are also used with silencers, so you end up with something that is the about same OAL as a gun with a 16" barrel anyway.