I am one of the lucky few who has had the DAVS550, no one else on these forum's seems to have had one yet. Mine was supposedly faulty as it always displayed subtitles in English and when you switch them odd they come back on possibly on the layer change in a foreign language, also miscellaneous messages kept coming up when watching films.
However, the sound quality was superb and the DVD video quality was very good through S-Video. I have only played around with the DAVS500 in Comet so cannot really compare them, sorry, however the speaker output has been increased over the S500.
If you think that the subtitle problem will bother you, check that it has been fixed before purchasing, this problem is also happening on the DAVC770 and DAVC990 in the US which would indicate they have a firmware problem. I have contacted Sony about this by Telephone and E-mail but as of yet they have not been very helpful, they seem to have completely ignored my 2 e-mails and on the phone told me 'Sony would not manufacture a fualty unit and the unit must be fualty'. As soon as I get a replacement I will post on these boards if problem still exists.
Happy hunting.
P.S. Pro Logig II worked very well with Rogue Leader on the Nintendo Gamecube.