I think it'll be close, mainly because of MW2's huge opening. (Said opening will not be matched by GT5.) However, in the long term, I do think that GT5 will outsell MW2, probably by about 500K. There's still not a ton of simulation racing games on the market (I'd argue more that the kart market is starting to fill), so it will sell well. I also see GT5's legs being much better due to the lack of games in that category; another good shooter will make MW2 almost forgotten.
-dunno001
-On a quest for the truly perfect game; I don't think it exists...