The thing about the Kindle is that it has a genuine appeal to people who like to read books a lot - like me! Whereas the iPad...well, it's not a dedicated ereader anyway, especially with that reflective screen. The Kindle does it for me because I completely forget I'm reading on a device when I'm using it - which is the whole point of a dedicated reader. I devoured the massive 'Jonathan Strange & Mr Norrell' (fantastic book, BTW - well recommended) within a fortnight on it.
Bear in mind, the Kindle has a big lead in sales owning to international distribution - AFAIA, the Nook is only on sale in North America. But I could well be wrong on this.