I've watched videos of both. I own a ripstik and they appear to be identical.
There are a few minor differences of the boards. Over all they are pretty much made of the same material. The Ripstik or Ripstick has a 1 mm smaller wheel, the decks are all solid colors, and the middle bar is slightly longer. The Wave Board has more wild designs of decks, 1 mm larger wheels, and the middle bar is shorter then that of the Ripstik.
They both ride the same and consist of mostly cosmetic differences.