It's more likely that I'd buy BC2 than MW2, I have MW2 already but personally I always liked the Battlefield matches. 1943 was really good, I'd buy a new Battlefield game based on that - I'd buy BF2142 for the 360 as well if they'd make it. I think that would be an awesome XBL game. I even like BF:MC, it's big and epic, and for a long time it was the best online shooter for the 360, people still play it and enjoy it.
All I've really played of MW2 is the spec-ops 2-player stuff, which is good fun, but I am not a huge fan of the campaigns in COD bar the first game. The campaign mode in the original COD was incredible IMO, but these days I rather play co-op games like Borderlands. If I want a plot, then these days I like it to be vague and just let me get on with it. When I play a plot based game, it tends to be an RPG, like Fallout3, maybe the best solo game I've played in years. Farcry2 was immensely good as well, great online, great map editor, and the story mode lasts for ages and affords a lot of freedom. I guess I prefer a sand-box style RPG shooter to pure shooters like COD.
I can see the appeal though, COD has an epic feel to it and it can suck you right in - personally I'm just a little sick of running through post-war, eastern european streets.