It is clear that the PRS-300 is a PRS-505 follow-on with a slightly smaller screen, and no removable media. For example it has 3 font sizes.
The PRS-600, on the other hand, appears to be an improved version of the PRS-700 with no side-light. In addition to all the improvements over the PRS-505 in the PRS-700 (including touch screen and 5 font sizes), it adds
- Freehand highlighting and annotation
- Built-in Oxford American English eDictionary
Both still have 8 greyscales, which surprised me.
The big remaining question is how good is the PRS-600's screen? In other words, how much of the PRS-700's fuzzyness was due to the sidelights.