Olly, as a hardcore gamer I would say consider the most important aspect, what does your kid's friends own? rather than considering any of the other aspects this is the most important one as then, they share games and they can play multiplayer for which they need to be on the same platform. So unless you are planning on a surprise gift, do ask your kid about this. coming to the other aspects these are my thoughts after owning both the xbox 360 and PS3:
quality of games - nothing between the two. only question is the exclusive games of the platform, now this is highly subjective given the taste of the gamer in question, but generally true fact - PS3 has better variety of exclusive games to the platform in terms of flavour, XBOX 360 is more first-person shooter driven.
recurring cost of ownership - nothing between the two. both the platforms have same prices, between 2.5-3k each game. The only thing to consider here is if you are in a position to buy cheaper imported games. Retail prices at current USD rates are actually higher outside than in India, but you do get cheaper pre-owned games in both US and UK from large chains, so if you are a frequent traveler overseas this can be a source of cheaper games. Now here PS3 has an edge as XBOX 360 games are region coded and PS3 games are not, so while buying XBOX 360 games from US one needs to do some research on whether it works on PAL consoles (the Indian version).
the casual motion gaming aspect - again here it is slightly tricky. There are few things to consider here:
cost - As far as the initial cost of ownership goes, Kinect is more expensive, but there ZERO recurring cost, whereas in PS3 you need to add move controllers for each incremental person wanting to join in. so on a recurring basis Kinect is more hassle free.
experience - depends on taste, although kinect is truly more user friendly and absolutely controller free, move is more accurate, all said I will say again nothing between the two.
available content - similar, doesn't matter which console you are going for. kinect does have some exclusives but they get old fast.
SPACE - this is most important. Kinect needs space. For a good experience you need at least 6-7 feet of clear space from the tv to where the user is, and this space should be clear, no furniture in between, if there are 3-4 players playing, u need 8-10 feet of clear space to be comfortably playing. PS3 move, on the other hand doesnt have this problem at all.
so those are the things to consider. In a single line, it doesnt really matter which one you buy as ypu understand from the above points, the only and most important thing is what your kids' friends own, otherwise he wont be able to play with them on multiplayer games.