If you didn't know, Halo 3: ODST was originally going to be a stand-alone expansion to Halo 3. That basically means that it wasn't originally intended to be a full game like it is now. ODST will only feature a new Campaign, but keep Halo 3's matchmaking. It is going to be a great game but if you are looking for a full game that is 100% new, get Modern Warfare 2. MW2 is going to feature lots of new weapons, drivable vehicles, and awesome environments. Definantly going to be awesome.
Both of these games are going to be amazing but if you want something entirely new, get MW2.