Similar to your question....I recently upgraded to from the Sony BDP S350 to the Sony BDP S370.
The main reason for the upgrade was to access iplayer and BBC HD through it.......but......
After watching just a few seconds of a blu ray disc I knew well, both my wife and I noticed a difference in the detail and sharpness of the picture......
It ended up being a well worth upgrade, especially as I bought the S370 for £100 delivered, and sold my S350 on at £50!!
Hope that helps....if not, take your player to a shop and ask for a demo of the S570/S770 and see for yourself...