Both the BD50 and PS3 are final profile, Profile 2.0. The BD50 may have better picture quality, although according to one review the DVD upscaling performance of the PS3 is better. What the PS3 is missing is any analogue outs and therefore any way to get HD audio to a non-HDMI AV amp. The BD50 has this, plus support for all HD audio codecs.
The other obvious difference is the PS3 is a games console, the BD50 is a blu-ray player only although it will of course play DVDs, CDs, MP3 from disc or SD card, video from SD card.