I can't believe I didn't know there was a bicyle forum here! (that's what happens when you don't scroll down enough...)
anyway, I know more about bikes than oil (I wrenched in shops for 15 years), so... I'll say what I've always told people about upgrading: you'll never regret it. get the most expensive bike you can afford. from entry level, up to a couple grand, there is appreciable improvements at every step, i.e., a $400 bike IS much better than a $300 bike. I'm not familiar w/ that giant model, but they are no different from others, in that the better ones really ARE better. no one ever came back to me saying they bought TOO MUCH bike, but plenty came back saying they should have bought the better one. instead of starting entry level, just get the better bike. it'll ride better, be more fun, and you'll want to ride it more.
as for disc brakes, they only work well when they are good. some mfgrs put cheap brakes on a low end bike just to hit a price point. IMO, discs should only be pursued if you can afford hydraulic ones.
one last point, make sure the bike FITS. most important.