If you don't have any familiarity with the M4/M16 manual of arms, and therefore nothing to unlearn for running the 556, I'd say it's pretty much a toss up and comes down to whichever one you like more. (Even if you do, there's not a whole lot that has to be done different, mostly just the charging handle.)
There's more aftermarket stuff out there for modifying an AR than a Sig, if that matters, though there's enough these days that you can make a Sig as tactical or tacticool as you want. Again, pretty much a toss up.
As for 1-7 twist and 55 grain ammo -- non-issue, in my experience, and the faster rate of twist will let you use better 75 and 77 grain loads if you've ever got a need or desire for longer range shooting or serious defensive carbine kind of work.