Mario Party 8 is an awesome game you should really get it. It is more fun than the game cube ones. I haven't put it down since it came out. If you aren't sure then you should rent it first. I am going to get this game when it comes out too.
i'd say get mp8 just because from what i've read, bba only has 15 minigames, whereas mp8 has at least 50. i would certainly like to have both, because bba looks like fun, but i will wait a while for it to drop in price.