It depends on what you're lokking to be a hard and violent gamer then Warfare now honestly if you want to hit the record books with the high score of the century for any game then Borderlands is the one wait I take that back if you like arcade games then Borderlands
Modern Warfare 2 is good but IW screwed over the pc community with a lack fo dedicated servers for multiplayer.
Borderlands is also awesome but not as refined as modernwarfare.
I would go with Borderlands because we don't really need IW anymore since they have introduced new concepts to fps multiplayer video games and therefore should step down and let other creators make some games and new ideas.