Pardon my ignorance, but what is the difference?
It you want to get into the details here is one good source:
http://www.avsforum.com/avs-vb/showthread.php?t=931796
Like most threads not everything you read are necessarily accurate but for all intents and purposes this thread seems good.
As a user, it may suffice to know that the FAT PS3 cannot bitstream (sending the raw HD codec data) to your AVR or pre-pro. It can, however decode the HD codec from the media disc/or server and send the decoded data in LPCM uncompressed format to the AVR/pre-pro that can do the rest to playback the HD audio without losing any information. The slim PS3 can do it both ways so you have the choice to let the PS3 decode the HD audio or bitstream the raw data directly to an AVR/prepro that has the capability of doing its own decoding.
From my own experience there is no discernible difference either way in terms of sound quality. Bitstream allows the AVR/prepro to display the codec so you can see whether is it DTS-HD MA or DTHD etc directly on the AVR/prepro's display whereas with PCM you have to press a button on the PS3 remote to display the codec on your TV screen. PCM has the advantage that it preserves the full audio content whereas if you bitstream as the instruction manual says "If Bitstream is selected, some portions of the audio content may not be output. ... ", not that it matters as most people probably would never pay attention to those secondary audio contents on some BR discs. In general if you have a high quality AVR/prepro bitstream may (in theory only) get you better sound quality but if you have an entry level unit then PCM may be the way to go.