Its all down to personal preference really but i'll stand up for the PS3 in this occasion
- More advanced than 360 technology wise
- Blu Ray player thrown in for good measure (if you're keen on getting it set up with your 42" screen and HDMI then you might as well be enjoying Blu Ray movies too!)
- Future proofed with HDMI V1.3
- Dolby 5.1 & DTS
- Wireless out the box (or wired if you prefer)
- Can be set up to access your PC or laptop wirelessly and use them as a media server enabling you to access your MP3 library or photo library etc.
- Free online gaming (as opposed to 360's subscription based service)
- Card reader slots for Compact Flash, SD Cards, and Memory Stick (maybe just 60gb version?)
- Bluetooth
- Decent browser to allow easy access to the internet
I'm not to sure where all these good 360 titles are though that people bang on about, the 360 has been about for approx 18 months longer than the PS3 and i wouldnt say it has 'tons of good titles' - the only good 360 titles that i can think of that are specific to the xbox are Halo, Gears of War and Project Gotham. And on the flip side the PS3 has titles like Metal Gear Solid and Gran Turismo, etc. These days most games are available for multi formats anyway e.g. Call of Duty 4, Grand Theft Auto etc.
Like i say its all down to personal preference - at the end of the day they are both good consoles and i'm sure your son will be happy with either.