Depends on what grain bullet you choose to shoot. For an upclose, CQBish rifle that you just want to run 55gr. XM193 through, a 1/9 would be fine. If you decide you want to build a precision/varmint AR, and will be lobbing bullets out to longer distances (where a heavier bullet would be preferred), a faster twist such as 1/7 would be preferred to stabilize the heavier rounds.
Just an FYI, sending a light bullet through a fast twist barrel can cause it to over spin, and sometimes even cause it to break apart in midair.