I would go Kindle hands down. My husband bought a Nook wi-fi with an expenive case, spare charger, cable and sd card because he did not want to wait for the new Kindle. We both hate it for different reasons. He just plain likes his paper books and no device will suit him. I don't like the longer size, heavier weight and the very annoying touch screen which I find hard to operate. It seems very unstable to me . I like the click buttons and keyboard of the Kindles.
I have a small Jetbook which is great except for night reading and dear husband naturally is trying to get me to forget the new Kindle 3 and use his Nook. No way!! I'll stick with my Jetbook until I get a Kindle.
We've put the whole Nook package up for sale on the fleamarket page
https://www.mobileread.com/forums/showthread.php?t=96882
at a really good price and not one reply, so it seems folks are wanting Kindle3 or old Sony 505s'. The Nook seems overly heavy to me and the page turn buttons do not line up with my fingers and also squeak. The other annoying thing about Nook is that it must be charged frequently, even when not in use.
I find I can easily send any epubs to my daughters' Kindle, so file formats is not really of importance to me.
The Nook is lovely , but not for everyone.