By looking at them, I'd say Modern Warfare 2. MW2 has alot of stuff going for it- It's present day war with present day guns, gadgets, aircraft, ex. Personally, I think there are way to many WW2 games, and I think Call Of Duty World at War was a mistake. Most gamers agree with me, so there is alot of hype for MW2 not only because of the time setting, but also because Call Of Duty 4 Modern Warfare was so amazing both in game play in graphics. From the released trailers, MW2 looks like its going to be just as good as COD4- if not better.
Now for Halo 3 ODST. I personally like the Halo games alot. I find that the are fun to play, and that the worlds are great to explore. But, with Halo 3 ODST being the 5th Halo installment (including Halo Wars), I would have to admit the series is getting kind of old. From what I've seen so far, I the story line has nothing whatsoever to do with Master Chief, and aside from that, it doesn't seem very exciting. From the trailers it looks like game play is going to be the same with possible a few minor changes. Don't get me wrong- the Halo series is great, but it looks like Halo 3 ODST is lacking any major scene or game play changes, making it pretty much the same as the other Halo games.
All in all, I would say that Modern Warfare 2's future looks extremely bright, while Halo 3 ODST is receiving considerably less attention. Still- neither of the games have come out yet, so it is too early to judge, but based on the available trailers and information available, I would think that Modern Warfare 2 seems to be a cut above Halo 3 ODST.
