I think that Mario Party 8 is considerably better than Mario Party 9 for a number of reasons. First, and definitely the biggest point for me, is that I HATE having to ride in the same car as my opponents. I loved Koopa's Tycoon Town from Mario Party 8, and a map like that would have no place in Mario Party 9 due to this strange gimmick. As for my second point, I generally prefer the greater number of modes and mini games that can be found in Mario Party 8, including the pretty cool "final boss battle."