The Astak
Pocket Pro & EZReader have the same screen type as the PRS-505, a faster processer, more filetypes, and the same ease of file transferal, plus innate folder support. (I'm told I could hack my PRS-505 to get folder support; I'm a bit scared to try.)
When I look to replace my 505, I'm looking at a Pocket Pro because I want the smaller screen size, but the newer EZReaders (Mentor, I think) have the same processor as the PEZ with the 6" screen.