They're fundamentally different experiences. "Better" depends entirely on what you want from a game.
Core Loop: Five simplified sports (Tennis, Baseball, Bowling, Golf, Boxing) using intuitive motion controls.
Pros:
Cons:
Core Loop: Improvise arrangements of 50+ songs by "playing" 60+ instruments via motion gestures. No scoring, no fail state.
Pros:
Cons:
Both are packed in with many used Wii consoles. Try both—costs nothing extra.