From a competition standpoint mw2 is a terrible game. Noobtubes were way overpowered, quickscoping was way too easy and no dedicated servers ftfl. Bo has it's problems, mostly terrible respawns and spraying aimlessly, but it is simply much more balanced on a skill based level.
This also depends on what youre wanting to play...online is the only way to go. If you play them for te storyline and such than just don't buy them...
All that being said, cod4 > all.