ps3 has better processor, video, etc. they also don't ban units for modding, so a used one off ebay is most likely good to go. they have less hardware issues, while the xbox is a little...TOO prone to the red ring. in fact, from a survey of gamers, FAR to many xboxes have failed (between 25% and 50%).
the ps3 has great exclusives, and the fragfx is much closer to a pc than an xbox will be, as the fragfx uses a mouse and other bits to be a great improvement over the controls. plus, there is the playstation move, which does stuff sort of like the wii, just not as badly as the wii. the xbox has games like gears of war 2, while the ps3 has stuff like killzone 2 and the darkness. the xbox is aimed at the more general/casual gamer audience, while the ps3 is aimed at more hardcore gamers. plus the ps3 has built in blu-ray, which saves the expense of buying a player.
xbox is cheap enough, and the ps3 slim is pretty cheap too.
I don't have either, but I have tried both out and I personally prefer the ps3. was still impressed by the xbox, it is quite nice to use.
oh and the ps3 version of xbox live is free. no membership required.