Because of the small screen, I would say the Wii VC. I hear they gave the game a graphics boost, so it shouldn't look too bad. Besides, the controls are easier on a full controller with thumbsticks.
The DS version does allow you to play as Yoshi, Luigi, and Wario though, and it has plenty of cool mini-games.
Still, I would say the Wii VC one. But how about both? I don't think it would be that expensive.