I agree that the new models are a big enough leap from the 505 to be worthy of an upgrade. I really like the touchscreen and the ability to swipe to turn pages. My favorite features of my 900 compared to my 505 are the built-in dictionary and the cover image display. Some people think the 7" screen has a funny aspect ratio (and I did too before I got one), but I really enjoy the extra length and more words on the screen. PDF viewing is better with the newer models too. I also like that I have the ability to crop margins, and the new models also have the ability to vary contrast. Lastly, I have to use an SD card to organize my book collection on the 505. Happily the device memory of the new models is so large that I don't need the SD card. Indexing of the books is also faster than my 505 (although still not as speedy as I'd wish).
If you are interested in the 950, in addition to the Sony store access, it will also have a web browser to give you access to Wikipedia, google and other sites. I was always envious that the Kindle has wikipedia integration, even though I usually have my phone nearby and can look info up quickly anyway.