I own both and would recommend the 360 anytime. The whole experience is better from navigating the menus to playing the online side of things. The downside is £40 a year for Xbox Live although that can be found at less than that with an internet search. For your £40 you get smoother online play, voice chat included out of the box rather than having to pay for a headset and the chance to block players you find obnoxious or sweary. However, at £80ish less than the PS3 you effectively get 2 years free online play.
Lips is available for the 360 which is their version of SIngstar, both are much or a muchness. As already stated, Rock Band, Guitar Hero and now Band Hero are all available which do the same thing, but with drums and guitar as well. All good clean family fun.
Call of Duty games are availble on both consoles which should satisfy your urge to shoot people in the head, but if you want Halo it's a 360 exclusive.
The PS3 has the Blu Ray player, but to be honest with an average 32" HD TV and a 360 Elite running with an HDMI port you'll struglle to notice the difference as that upscales your normal DVDs anyway. The PS3 has just got iPlayer support , whilst you can now have Skyplayer on your 360 if you already subscribe. The PS3 has wireless compatibility out of the box as well, whilst the 360 requires Microsoft's adaptor - best bought from ebay as it's cheapest there.
The choice is yours, but I would recommend the 360 every day to anyone