ODST was a good game, and if you have enough friends to enjoy firefight then you will have fun, but halo wars is a longer game. Depends on which you like better, rts's or fps's.
really if you have friends who will play enough odst is a lot of fun, other than that it ends too quickly, although the story is still good.
Basically if you have enough friends and don't mind a short story, ODST
if you want a longer game and like rts's Halo Wars