There was a time when there was a big difference, however now, technically they are pretty much identical apart from one key issue for me; console noise. My PS3 is much, much quieter than my 360 which is a huge boon when it comes to media playback. I wouldn't say the noise from the 360 is anywhere near enough to write it off as a media player but if a person had a choice i'd recommend using the PS3 for that reason alone. There's also the fact that you can transfer video files to the PS3 hard drive and store them there permanently, last time i checked the 360 couldn't do this, again it's a small issue though.
In short, there's very little in it, both are very good media extenders.
