i have an rigid (no suspension) aluminum frame trek and I love it, make sure you do some riding on whatever you're looking at before you buy it and make sure it feels good to ride. My brother has a really nice schwinn with disc brakes, suspension and the whole works but i hate riding it because you lean too far forward on it, he's even taken some spills over the handlebars. disc brakes are nice until they get dirty or out of alignment and then they squeal like hell. look at tire tread, if you're going to be riding on pavement a lot you won't want a really knobby mountain bike tire as it wastes a lot of energy, not really a big deal because you can always buy a second set of tires and switch them as needed. Not to knock you PT, but everything i've ever had from kmart, wal-mart and the like has been lousy for anything other than casual occasional riding, go to a decent cycle shop, they can probably give you a good idea of what you want.
And most importantly, whatever bike you get, maintain it, i forgot about that last year and now i'm thinking that i'm going to have to replace most of my drivetrain.
Hope this helps