I recommend the Kindle, because of the nice interface of the Kindle(s) and the great Amazon customer service. You can return a Kindle book if the formatting is bad or something, but no other ebook store lets you return for any reason.
I would personally recommend the Kindle Keyboard, with 3G, if you think your wife would like to be able to get books when she is away from wi-fi. I love being able to get a new book while waiting at the doctor's office, etc.
Edited to add: I don't really know about library books and the Nook. I do know that they have to be delivered via wi-fi on the Kindle, or downloaded to your computer, and copied to the Kindle (no delivery via 3G).