I'm assuming you own both consoles. If you don't, obviously go for the one you can actually play.
The Wii game is bound to be cheaper, especially pre-owned, so if you can find that for a good price, go ahead and buy it. The new features in the 3DS version aren't too outstanding. There's a few new levels at the end of the game, as well as an easy mode with some new items. Despite these little additions, if the price difference isn't that great, I say get the 3DS version, anyway. The lesser display can be said to be made up with the portability factor. Take into consideration what features here you care about. If you're out and about a lot, definitely get the 3DS version if you desire some DKCR on the move.