In Brawl, your skill level never stops growing. In Mario Kart, your skill level will stop growing after a while due to the fact that you can only get so good. After that, you just cruise into first and nobody ever passes you. You can't really get any "better" than that, can you?
Brawl has a lot more variety. Mario Kart is just race, race, race. It's still a great game, but sometimes racing gets old. Brawl, however, doesn't get old, because every opponent is completely different; and therefore every round is completely different.
Brawl seems like a better choice because there are new characters, more things to do and unlock, overall just a really fun game compared to Mario Cart. Mario cart is a fun game too, just isnt AS fun in my perspective.