The 505 is a great device and I kept it for my wife after upgrading to the 600. IMO Silver is the way to go
It just feels the most contemporary to me.
I loved the 505s "hackability" and screen, but love the extra pdf-functions like zoom and pan of the 600 even more. If Sony were to make a new device, I would love to see the "605" having the same readability as the 505, the touch thingy (finger and stylus), PRS+, maybe a calendar, File organization on device and a faster, much much faster processor.