1 in 7 hands down. There is really no down side whats so ever to 1 in 7. It will stabilize everything from 45gr varmint rounds all the way up to 77gr MK262 rounds.
With a 1 in 9 you may not be able to stabilize the heavier rounds which not only have superior external ballistics, but also have superior terminal ballistics.
Also make double damn sure it has a 4150 steel barrel that is HPT/MPI and a bolt that is HPT/MPI and shot peened. I would look at a carbine from Daniels Defense, Colt, LMT, Bravo Company, Noveske, or Sabre. These brands have a reputation for reliability and durability. Steer clear of DPMS, Olympic, and Bushmaster.
Anything particularly bad about dpms and bushmaster?