That was hard to keep straight. According to The Unofficial Apple Weblog (TUAW), Barnes & Noble's nook is beating out Amazon's Kindle, but the iPad is outselling both. Well, good.
I have a nook. I don't have a Kindle or an iPad. I'd like them. I read Kindle books on my iPhone.
In the grand scheme of things, I don't see any doubt that the iPad has the most elegant user interface (UI). It also has the most function. Not only can you read a book, you can write a blog post ABOUT the book on the same device. Technically, you can do that on an iPhone as well, but it'll be more challenging.
I would say as a runner up in elegance, nook comes in second, although I've never physically touched a Kindle. I've never seen one up close. I've seen plenty of pictures and unboxing videos of them, but never seen one in person. Nook is very streamlined and pleasing to the eye. It would be #1 if the iPad didn't exist.
But when it comes to any kind of eBook reader, content is king. I don't care how cool your iPad or nook is if you can't read anything on it. At the present time, Amazon is the king of content.
I've bought a few Kindle books. I haven't bought any books for nook yet. Both Amazon and B&N offer free eBooks. B&N's don't seem to update very often though. I got my nook on January 15. At the time, B&N offered 2 books for free in a Star Wars fiction series. I downloaded them. Amazon offers those two plus book 3, which has yet to show up on Barnes & Noble. I've seen very little change in Barnes & Noble's selection (at least their free selection).
What my nook is good for is reading pdf files. I download a lot of free pdf documents and rarely get around to reading them. I haven't found a good (free) way to read pdf documents on my iPhone, and my netbook's battery life is pretty bad (2 hrs). So I put them on my nook. But for books, I typically use Amazon.
I would love to see a universal content. Then you could buy the reader that you want, and buy the content that you want, and join them together. I don't like the current model where the reader you choose will dictate the content you can buy. The latest software update for nook allows web browsing and playing chess. Whoopee! I'd rather be able to read my Kindle content on it.