one consideration could also be the price of the books you want. Not the availability of free books and such but the kind of books you actually want to read. For example the books I want to read were generally $1 or so cheaper on Amazon than on Barnes and Noble. I can go through books pretty fast so it all adds up.
And yes I have exchanged books with a friend that lives in Angola. It's great. I have the one with free 3G.
I haven't really had a need to read in the dark so no opinion on that.