I had a chance to compare BD-C6500 against PS3 Slim last night. In short, Samsung wins hands down when it comes to video quality... from any source (hd, dvd, avi). PS3 wins hands down when it comes to streaming software (ps3 media server vs. pc share manager).
Quality of the video on PS3 pretty much matches the quality produced by ATi 4200 on my HTPC. You could say exactly the same.. a OK. However picture produced by Samsung is truly amazing... there is a certain liquid quality in colors coming from the c6500... heavy, smooth and clean if not a bit too shiny. Clarity and sharpness at the same time are also present. Really good work by Samsung. In comparison, color quality on PS3 and/or HTPC almost leans toward the crayon like feeling.
Quality of the software for streaming... well there's really not too many good points or features on Samsung's side except for the fact that when it streams it is rather stable. PS3 media server one the other hand is also as stable. But feature wise it's really a better tool: no (pre)indexing of the content needed, video navigation capability is awesome, it presents all titles from backed up dvds (iso or folder) as titles and not as sets of vob files, it literally handled all the files I had in my repository (pc share manager didn't), allows access to multiple audio tracks, no meta crap leftovers after indexing etc.
One more thing worth noting is that Samsung supports soft subtitles with any content. PS3 doesn't so what PS3 Media Server does is transcodes the video overlaying subtitles on it before streaming it all to PS3. That's really a big win for PS3 Media Server, can't do better than that, but picture quality does suffer and wireless connection in such cases is useless... as bit rates climb to 100Mbits. If you don't need subtitles then remuxing works just fine.
The comparison was run on the 1080i CRT, Sony WEGA. Same HDMI cables were used and the same HDMI repeater switch (Yamaha 6240). Meaning, picture setting on the TV was one and the same.
Last but not least, Samsung runs dead quiet and cool... which is anything but when it comes to PS3.