Depends what you're looking for. GC Paper Mario is closer to a straight-up RPG with some platforming elements in-between fights. Super Paper Mario is more of a straight platformer with generous amounts of RPG elements thrown in. Basically, it's a choice between menu-driven fighting with platforming breaks, or a platforming game with item menus and NPCs to talk to? For what it's worth, I enjoyed SPM a fair bit, but was pretty lukewarm on GC Paper Mario. That's just my personal preference showing through, though.