I have used 6inch Sony reader and 5inch PocketBook 360 and I strongly prefer PB360.
The screen has much finer resolution, so it can bear smaller letters while maintaining readability. On PocketBook you can fine tune the look of the text on the screen. You will spend first couple of weeks trying out lots of combinations of various fonts, line spacings, justification, font sizes...
I can squeeze more text on PB360 screen that I could on PRS-500.