The Xbox 360 was released in the UK in December 2005 which is 15 months ago (come on Rick keep up), the PS3 will be released on 23rd March this year. On thing this means is that there are already tons of games for the 360 but you will limited to a handful of release games for the PS3 until more titles are developed.
The 360 is now priced at £280 for the premium version (the only one worth buying) whereas the PS3 is £425 which seems a lot but you get all the functionality of the 360 plus free online gaming (you have to pay a monthly subscription for online gaming with Microsoft - there's a surprise!) and a blu-ray dvd player. This is one of two new standards in high definition dvd the other being HD-DVD. Bear in mind though that blu-ray has only just been developed and therefore the PS3's player is likely to be far superceded by stand alone blu-ray players in the coming months/years so buying the system for this reason is probably not recommended.
But hey, perhaps he just wants to play games!!!!!!!!!!!!! And you can do that too
I have an Xbox 360 and I'm very impressed. The range of games are good and the graphics are (for me) much better than my old PS2. I haven't seen the PS3 so I don't know how much better it is. Bear in mind that the gaming side of the PS3 has been developed for quite a while and the delay in release has been due to a shortage of blu-ray components so it is unlikely to be two years ahead of the 360 in terms of graphics and game play.
It's worth saying that both of these consoles have high def outputs but as I don't have a high def tv I can't comment on how much better things look that way. The PS3 is actually capable of outputting 1080p compared to 1080i from the 360. The difference being that on 1080i you get 1080 lines on the screen and it draws every other one each time whereas 1080p draws all lines at the same time meaning the display should appear smoother. Not many games have been developed for 1080p at the moment though.
It really depends what he wants. If he wants a very good console with lots of games now then I would recommend the Xbox but if he wants something that will become better over time then I'd go for the PS3. Also it's probably important to decide if he wants to play online against other people a lot as Xbox live is £39.99 for 12 months whereas PS3 is supposed to be free.
You may still be able to pre-order a PS3 for release date but if not it's likely you will have to wait quite a while to get one