Welcome! I started with the Sony PRS505 too, and still have it, but earlier this year decided to try Nook. I currently have the Nook touch, but Sony has a new version of reader called the T-1 that I would probably have bought if it had been out when I was shopping this spring.
Some of the big differences between the 505 and this years's releases include: speed of processing, touch screen, wi-fi capabilities. But hang on to your 505 as long as you're comfortable using it, there's always bigger and better right around the corner. These days you can include "cheaper" too.
Vic