If you still have the GCN version, then you're better off sticking with that one.
Everything you loved about the original GCN version is still here: the crazy and wacky power shots, the wacky and crazy item battle matches, the Mario stars that you all know and love such as Mario, Luigi, Peach, Daisy, and many more, and even the fun and gimmicky tennis courts like Luigi's Mansion and Wario Factory are here as well. But unlike the GCN version, where you had to just simply press A and/or B to hit the ball, the Wii version requires the player to swing the Wii-remote. Now to be honest, I was kind of disappointed with this addition. I wouldn't have minded it if you could use the Gamecube controller as well, but unfortunately the GCN controls are NOT present in this game. And having to repeated swing the Wii-remote just to hit the ball can get quite tiresome after a while.
Other than the Wii-controls, there isn't anymore added from the GCN version. Now this can be a good thing or a bad thing depending on how you look at it. But this can be a turn-off for some people if you were expecting something new that wasn't in the GCN version.
So to sum it up, Mario Power Tennis is a great game, but unfortunately it suffers from tiresome and uninteresting controls. If you don't mind swinging the Wii-remote over and over, then this game is definitely worth a go. Otherwise, you're better off just dusting off your GCN and playing the Gamecube version.