Good question... The Xbox 360 has more and better games at the moment and even graphics are so far superior to PS3 games. It is on the other hand more closed, so dont expect to run linux or homebrew on it. It has also had some reliability issues, while the ps3 seem to have ok reliability stats.
The xbox 360 also has better media playback ability currently including divx/xvid upto 720p resolutions with 5.1 ac3 audio. Sony has also announced a divx update for the ps3, but there is sadly no info on what res/bitrates it will support and if it will do 5.1 audio.
The main benefit of PS3 is ability to use linux and play blueray movies.
