I do love my 505, I have only had it for two months and I can't imagine life without it. Living in the UK, Kindle wasn't really an option. I looked at a few others, but went with the 505 because I could buy it a local Sony shop on sale.
I also wanted a device that could read epubs as well as a reasonable variety of other formats.
The main thing I want in an upgraded Sony 505 would by a wireless function.
Have you checked out the 'Which one should I buy' section?
https://www.mobileread.com/forums/forumdisplay.php?f=123