To me, the only twist to consider for an AR carbine is 1/7. The only exception to this is a person that wants to shoot 35-40 grain super light bullets for some specific type of hunting, and then they are likely getting an AR rifle with 20" barrel. A 1/7 twist will stabilize anything from 50gr Hornady VMAX all the way to 77 grain heavy loads for long range and self-defense.

Of course with the S&W lineup, you are limited. Reports suggest that the Sport's 1/8 barrel is pretty decent, but with that model you have no dust cover. The ORs and other standard models have a 1/9, which is a disappointment.