Modern Warfare 2 is coming out in November and Halo ODST is overall, a campaign expansion. It has campaign and firefight on one disc and Halo 3 multiplayer on the other. So unless you want to be playing campaign at the LAN you are essentially going to play Halo 3 with a selection of more multiplayer maps.
Also I don’t care either way about default or MLG as it seems a great majority of the tournaments are default. I was just wondering why you are leaning towards not having MLG.