My opinion ( sidenote, MW2 is not out yet, so no accurate comparison can be done)
Campaign-
ODST: Essentially a rehash of previous Halo games. Whatever your feelings are on the old Halo campaigns, expect it to the same with ODST.
MW2: The assumption here would be that it too would be a similar game to it's original title. It will most likely add several new weapons and several new vehicles. Which is something ODST did not do.
Multiplayer-
ODST: I haven't played FireFight yet. But the general consensus is that it is a load of fun. The game also includes 3 new maps for Halo3, so if you like Halo's multiplayer than it will most likely please you to buy ODST.
MW2: The assumption here would be that it will be exactly the same as it's predecesor, with the exception of all new maps, new weapons, and a probable new type of multiplayer game type being added. Again, if like MW's multiplayer, this will probably be the game for you.
In the end I have to say it's a matter of personal preference, primarily of the multiplayer systems for both. Me personally, I love Halo's multiplayer, and I hate MW's multiplayer. Ultimately, we know that both campaigns will look good, and be fun to go through, but it will be the multiplayer that really makes us love the game. So, to break that down for you, my opinion on the two styles.
MW & ( probably) MW2: This multiplayer is fast paced, and lacks a certain amount of strategy, which is made up by it's need for skill. In MW, a single headshot with virtually any weapon, will instantly kill you. Along with this, is the map, which constantly informs you of everyone's location, and more skilled or experienced players will have access to special weapons that you might not have access to.
Halo 3 & ODST: The speed of the game is slower, although it requires a little more strategy and an even amount of skill to play. The shields in the game force constant accuracy to defeat an opponent, versus the single headshot approach that MW took. The radar is limited, and can be easily rendered useless which forces more stategy to safely navigate the game. The game is an even playing field, with no person having access to anything that other people can't get, skill is the only advantage in this game.
The end is what do YOU want. uber realism or spacey fun.