I don't ever need to sync between devices, I read all the way through on one reader only. I despise social networking and think it's the scourge of the 21st Century
(and let me apologize now to anyone I might offend with that comment). I don't need the reader to tell me how long I've been reading a book, or how long until I finish a chapter or a page... all of those features are just totally useless to me. If I'm interested in reviews or checking Goodreads, I only do that from my PC, never from my reader. I just want to sit and read and not be bothered with all the other minutia they keep adding to readers.
So that leaves just availability and price as my only concerns when buying a book.