I picked up Resistance 1 and 2 at the same time when i picked up a PS3 a few months ago, and played them one after the other.
I found Resistance 1 to be a nice long game, as others have said, it made a nice change to know you got good value for money waith so much content.
That being said, i found Resistance 2 allot less repetitive (not saying it wasn't mind) and far more polished. It felt like they had turned the action/cinematic parts up a notch.
I'd pick up both tbh, they can be had for peanuts now anyway.