Call me the queen of backtracking, second guessing, and mind changing, but the PRS-600 has ultimately won out as my favorite reader.
At the end of the day, the 300 was too slow, the PB360 was too clicky (I read the NYT every day and as I'm scrolling down the menu it's click-click-click -- especially annoying on a quiet Sunday morning over tea / coffee), the Mentor was also clicky (but did have a very nice, clear screen).
Here's my ode to the 600:
https://www.mobileread.com/forums/showthread.php?t=96189