I have a PRS-700 and got today a PRS-600 for my wife.
There is a super cool function in the 600 firmware which I'm missing at my 700 all the time!!! I really hope Sony will bring this funtion to the 700 too!
It's if you are using the font size "M": The 700 has the problem that the printed page now is larger than the screen and you have always some lines on the next page. The 600 is able to show the beginning of the next page direct below the end of the page before! Full pages of text all the time!
THIS function for me is much more important than having a dictionary.
Sorry for my poor english. I'm from Germany and bought both readers in the USA.