GET BOTH! THEY ARE MY ABSOLUTE FAVORITE GAMES FOR THE WII!
Super Smash Bros. brawl is great for multiplayer games, but also keeps you occupied with the Subspace Emissary, and the one player modes, you can also take snapshots, build stages, and take video clips of home-run contests and target smashes...it's just an awesome game, I would say this and Melee are the bets in the series.
Super Paper Mario is definitely the bets in the series, it's incredible, it keeps you sooo into it, the plot is sweeet! the action is non-stop, you get up to 11 partners and secret ones! you don't use a lot of wii sensory(which is probably the best thing in ways of control), you travel through 8 different worlds, plus two main worlds(Flipside and Flopside), and there are tons of things to keep you occupied: finding cards, playing in the undergound arcade, BOTH pit's of 100 trials. it's non-stop fun! I would say this and the first one are the best in the series, I was somewhat disappointed with "The thousand year door", but I did like it)
what I ahve to say about both games is this:
they will be a lot of fun no matter what, but I think they will be that much more fun if you've played the previous 2.
personal experience, I own both.