Depends upon your tastes, really.
I think ODST's single player and cooperative modes (including Firefight) offer a lot more replay value than CoD:MW2's; if you're in it for the campaign, ODST might be your better bet. (I base this largely on CoD's AI engine, which is strongly scripted; once you've played through once, you can pretty much call what's going to happen on every subsequent play-through. Halo's AI mixes things up a lot more so replays are less predictable and more emergent.)
If your eye is more towards the multiplayer, though, ODST's is identical to Halo 3's with a few new maps added... if you're pinching pennies, MW2 probably offers the bigger return for versus play.
-- Steve
I think ODST's single player and cooperative modes (including Firefight) offer a lot more replay value than CoD:MW2's; if you're in it for the campaign, ODST might be your better bet. (I base this largely on CoD's AI engine, which is strongly scripted; once you've played through once, you can pretty much call what's going to happen on every subsequent play-through. Halo's AI mixes things up a lot more so replays are less predictable and more emergent.)
If your eye is more towards the multiplayer, though, ODST's is identical to Halo 3's with a few new maps added... if you're pinching pennies, MW2 probably offers the bigger return for versus play.
-- Steve