Welcome to MR, Asphodel.
I have the Sony 505 and love it. I like that I have a wide choice of where to buy or get free books for it. I also like the fact that if I want to "borrow" books from the library all I have to do is download to my 505 (well, after the standard of getting a card etc.)
If you are still trying to decide, remember that the only best device is the one that fits YOUR needs.
So, having an idea of : genre of books you like to read, whether you have the have the NYT best seller right now, if you prefer to have Classics - PD, CC, whether you need to read a lot of only pdf formatted books, whether you care if the books have DRM or not, etc will help in making your decision.
Have fun reading through the various forums, ask questions, and just have fun. I hope you find the best device for YOU.