I voted for the original one, just because I felt the dance moves are easier to follow. However, on the new one I like that you can go online with your wii and download new songs. I haven't done that yet, so I can't give details on it but I think it's a feature I may be interested in.
Also, on the second one they keep track of calories burned, and have more modes - dance off, etc.
But again, since you don't have either one you may want to get the first one first (and you'll likely get a better deal on it), and then get the second one for a birthday.