I've got all three, and couldn't split either way.. Here's my pros and cons for all.
Pros: Free online gaming. Excellent media playback w/ software like Nero MediaHome. Works with Bravia tv remotes. Doubles as a PVR w/ PlayTV. Brilliant sound definition, works amazingly well with a surround sound system. Amazing list of compatible devices; mice, headsets, keyboards, wheels, joysticks.. the lot. It's very, very quiet. Little Big Planet.
Cons: Load times are very slow compared to xbox. Graphics aren't as good as the xbox. (can't work out why, considering it's theoretically got better hardware). The control is much lighter, and delicate that the xbox control. The triggers are just nasty. Very sloppy.
Pros: Graphics & loads times vastly superior to PS3. Xbox Live is a much better service than Playstation Network/Store. More content, faster. Xbox control is much more versatile, better suited to racing & fps.
Cons: Microsoft official accessories are required; wireless networking, steering wheel, joysticks etc. All of which are horrendously overpriced, and particularly in the wheel department, poor quality. Xbox Live costs you money! Sound performance is not nearly as good as it should be. Media playback is a clunky, inconsistent mess. Sometimes it works really well, others not at all. It's bloody noisy! Gives my PC a run for it's money in the noise stakes!
Pros: It's cheap, and lots and lots of fun.
Cons: No 1080p support, so it looks crappy on a good telly.