I own both a Nook and a Kindle (because they both use different formats). I use and enjoy them both. The biggest advantage the Kindle has is the free 3g. I was very annoyed when I first got the nook because - to register it - I ended up walking down my neighborhood looking for someone who had an unsecured network. If you don't have wi-fi, you just can't register it very easily.

For both the nook and kindle, you can download a book to your computer and transfer it to your device. For Kindle, I have the free 3g, so when I buy a book it downloads to it directly.

I love reading on e-ink so much that I've grown to resent paper books. I won't buy them anymore. (Well, there are some exceptions - especially for books with illustrations). I was not an early adopter - I though people were nuts to prefer their kindles to real paper. But at my age? It's so much easier to read from the nook and kindle that it's not even funny!