It would depend really on what you're more interested in. Maybe you should read up on them. Which sounds the more appealing? Are you more partial to 2D or 3D games? Would you rather around, jumping on enemies' heads, solving puzzles and gathering stars, or would you rather spend your time wandering through dungeons, battling and gathering Pokemon?
Personally, I would recommend Pokemon Mystery Dungeon: Explorers of Darkness. It is by far the best Pokemon game to come out in recent years. Have you played the first set of Mystery Dungeon games? If so, you'll have a fair idea of what to expect...but then add on the 4th generation of Pokemon, change the storyline slightly, as well as new dungeons and extras like Pokemon-specific items, and you've got this game. It is very, VERY addicting. There is lots to do, and I think it would last you a lot longer than Mario 64 would.