I have owned and played both the PS3 and XBOX 360, and I would have to say the PS3 is the way to go:
The Playstation 3 is great because it has free online, chordless network capabilities, great exclusives, blu-ray dvd playing capabilities, great hand-sized controller, and the ability for games to be in HD with an HDMI cable.
The XBOX 360 is a great system from their great exclusives, Left 4 Dead 1 and 2, plus Gears of War 1 and 2, soon to be 3, amongst them, and being able to play games in HD if you have the adapter switch.
Trying to play my 360 online was usually a hassle. You needed all of those chords, and if the internet was a four star quality cable modem, things would lag, because it had to be five star instead. Another thing about my 360 that I hated was how loud it was when it ran. I had to turn my TV volume up loud, just to get over the loudness of it running. Plus every once in a while a loud grrring sound would come out of it, like it was getting ready to die right there.
In my opinion, the PS3 is the way to go. From its blu-ray playing capabilities to its great exclusives, free online, and chordless network capabilities, I think the PS3 is a great buy.
The hardest thing for me to wrap my head around was when someone said the games on the 360 looked better on that than the PS3. I would have to disagree. Having played both the PS3 with an HDMI cable, and the 360 with the HD input, I have tried: Call of Duty: Modern Warfare 1 and 2, Resident Evil 5, Just Cause 2, and Saints Row 2 on both the PS3 and 360. In all the times I played the games on both systems, the PS3 version always looked better.