Janie I would have to say its a drama, the fact that only the town survived is hardly enough to justify sci-fi.
Also the way it gets so involved later with family, relationships, and blah blah blah, it lost me during the season, I was one of the few that watched every episode and was not surprised to see it canceled. What surprised me is the brought it back.
There is a bit of action at times, but it is nothing on the ranks of that of 24 or PB. The scenes of any real violence are often tame in comparison to todays tv standards.
I say move it to drama.