You can give them the feeling of being on the cusp of the next "big thing" by going with Kinect which does have some decent games so it's not totally hype riding to go for it. Games like Dance Central, some of the first party stuff like Kinect Sports and fitness games like Your Shape would be good buys for it.
Going for a Wii there is definitely the widest array of games as being motion centric for the past four years offers certain advantages. It has a popular dance series going for it with Just Dance as well as fitness titles and there is definitely some mixing and matching that can be done with motion based titles and traditional games that use it to some degree as well.
If they already have a PS3 then going the Move route will likely be the least expensive one, the problem though is that outside of Sports Champions and MAYBE Eyepet if they have kids or wouldn't mind playing that kind of game, Move really has no standout software that really defines it. Even some of the more "gamer" style games it has, while good for what they offer, still don't really have as much on offer as say the Wii library has if that is what they're looking for.
In this instance I'd say it's an issue of cost vs. content. Going for Move would be more cost efficient but doesn't really have much in the way of compelling content whereas with Kinect and especially Wii there is a bit more on offer that helps make those devices stand out beyond just the utilization of their technology. With it being the holidays and all and the chance for group gaming potential being high, I'd also knock a few points off of Move for lacking in that department as well.
