Both MK and GTA have massive legs. Its going to be close in the long run but I think GTA4 will win.
GTA is both hardcore and casual at the same time. You cannont sell 20 million copies without appealing to a more casual type of gamer. And for those people that don't believe me about the legs of GTA. Look at this
| Take 2 | 182 | 14,290 |
It sold 14,290 in america this week. not bad is it?