New phone advice?

It's been two years since I bought a T-Mobile G1, and my contract is almost up. Should I get a G2, a Nexus S, or something else? I'm pretty attached to the physical keyboard on my G1, but I'm actually leaning toward a Nexus S, just because it's newer and shinier and I am somewhat fascinated by the idea of Near-field communication (not that I have a good use in mind... non-networked multiplayer games, maybe?). Does anyone have a good reason to prefer one over the other, or to prefer some other option? I'm pretty sure I'm sticking with Android, though. I'm also probably going to stick with a T-Mobile contract rather than get an unlocked phone—it wasn't too hard to stick with this phone for two years, so I think I should be able to survive another two-year commitment. But is there something on the horizon that will make me regret it? Or, is there some other carrier that I would much rather be on? I've had pretty good experiences with T-Mobile, though I don't have anything to compare it to. I am slightly wary about the advertised "4G speeds" rather than actual 4G, but is it worth switching to Sprint for that?