Here's a question for you..
Does your receiver decode DTS HD and Dolby true HD via HDMI? If not and you're primary source of input for these codecs will be multi-channel analog outputs, then get the Panasonic. Why you might ask? Well, with Panasonic you can tell the player in the menu one time that you would like legacy Dolby Digital/DTS to be output over spdif in either bitstream or pcm format allowing the receiver to apply any of the room correction it may be able to do or bass management. With the Sony, you have to go into the menu everytime you switch between a blu-ray with one of the newer codecs or an SD dvd with one of the legacy codecs and tell it to output over spdif because it won't do both at the same time.
So basically, with the 550 which I kept for about a week before I went mad, if I swapped in an sd dvd I had to go into the player's menu choose output bitstream over spdif and then play the movie. Want to watch a blu-ray? Go back into the menu and tell it to output over multi-channel analog via PCM. The Panny 55 will do both and will not require any additional menu changes to do so.
Now to be fair, this only matters if you are using the multi-channel analog outputs. If you have an HDMI equipped receiver that can decode the new formats or receive them via PCM this is not an issue.
Also, the new Sony's won't do a frame by frame advance when a blu-ray is paused...small issue for most probably, but some people find it very annoying.
