If you've got it down to two..........nook and kindle...........toss a coin and do it.
nook is a little more versatile as you can hack it and such. Also it'll read epub files which the kindle doesn't. There are a LOT of books in epub format. Personally, I haven't done anything with the library thing because of all the books out there. All the classics are free.
Then again...............there are a few sites that carry Mobi format ( kindle ) and you can sideload them ( plug your reader into your computer and drag and drop ).
For me, the best free ebook sites are: Smashwords, Deisel, feedbooks, and a few others.
For reviews on what you are currently looking at for ereaders go to:
http://www.the-ebook-reader.com/
Hope this helps.