I'll offer my 2c although I've only owned a PRS500 and 505. If you can get access to the Sony eBook Store this is a real benefit. They have a lot of newer books you wont be able to find in other formats (except from Amazon). I'm sure you can find a helpful MobileRead user to buy a Sony gift voucher so it is really quite easy to get an account on the Sony Store.
Both the PRS505 and the PRS700 read ePub (which I predict will be of increasing importance). So unless the built-in backlight is worth $205 dollars for you, I'd go for the PRS505.