I own both, and have to say that the Wii version could have been presented better. This really is a DS game that was ported to the Wii, and they should have put more work into redoing the second screen interface instead of just positioning it off to the side and killing a bunch of screen space on the TV. If you have a large enough TV this isn't that big of an issue, but still something that could have been polished up.
Having Said that, I enjoy the DS version very much. The game play and combat reminds me of the Mana series. The story line is decently done, and the magic system, though it takes a little getting used to, is innovative and quite functional if you pay attention to what you're doing. The game really benefits from multiplayer, as the AI on whatever 3 characters you are not controlling is not exactly going to win any awards.
If you find it for less than $30 I recommend picking it up, on either system.
This is a screenshot of the Wii version. Note the wasted landscape:
