If you don't own any systems and are looking to get one specifically for motion based play, your best bet is to go with the Wii. It's going to be the cheapest, and has the biggest backlog of motion-based games. It also has the best integration of motion and non-motion based games (i.e. games that you can play with a controller and buttons, but can still wave the sticks around to do fun things). The Wii is also generally best for casual gamers and younger kids, and is great for party play.
If you want to get serious about your gaming, and are merely leaning towards motion-based games, go with the 360. On the motion side, the Kinect is the only true controller-free option of the three. Plus, the 360 is HD (Wii is low def only) and will open up a larger world of gaming to you. And the 360 has what I consider to the be the best network (Xbox Live). However, this is going to be a more expensive option, as it will cost a lot more to get the full Xbox setup and add the Kinect on. If you choose to get Xbox Live (not required), it will also cost you a monthly fee. Only go this route if you want and are willing to pay for the full gaming package. Don't go this route just to get the Kinect. OTOH, if you already have a 360, this is by far the easiest (and probably cheapest) option.
The PS Move is the one I'm least familiar with of the three. Every time I look into it, it seems like kind of a middle ground between the other two; it's fairly blah by comparison, and has no real benefits over either alternative. If all you want is motion-based gaming, the only reason to get the Move is that you already have a PS3. There's plenty more room for discussion the PS3 vs Xbox, but if you're getting it strictly for the Kinect vs Move, I think the Kinect wins hands down.