My wants and needs are similar to yours. I've had my sony 505 now for a little over a year and I still love it. It's easy to use for library books (which is my main source of "free" books), but I have also found many places (including the sony ebookstore!) where I've downloaded
free books to read. I only use it for fiction in epub, pdf and sony formats.
I'm not as familar with the newer sony versions after the 700, but I know the 300 is similar but smaller screen. My 505 fits into my purse (I actually got a slightly larger purse for that purpose), so I am never apart from my books
Good luck deciding!