The only one I know anything about is the Kindle. My DH got one when the first gen came out, then he got me the Kindle 2. We both love our Kindles! My favorite thing about them is that all your e-books are stored with Amazon. That way, if anything happens to our Kindle, we can just download our books again to a new one. No repurchasing or hunting our own hard drives for our books. It's like having our own huge library off site! It's really nice! No worries if anything happens to our in-home storage device! We can also share books, so if there's a book he downloaded, I have access to it as well.
I also like that you can adjust the type size! That way there's no eye strain from trying to read tiny print. Some books are like that and difficult to read, but I'm not where I need to buy the "large print" editions! You can download e-books from other places as well, but those won't store to your Amazon library, you have to store those on your own computer.
I have no experience with the other two e-readers, so I hope someone else here can give you some insight to those!
I do love my Kindle and can't believe I was making fun of him when he first got his!
Good luck!