I’ve used both and prefer the Nook. The big reason is because it uses epub format for books and not Amazon’s proprietary format. This allows you greater flexibility to side load books and “buy” from more sources.
To add to my own post, I am using an iPad now which i prefer over both. The ipad also uses epub. However, if you are going to read outside, then you will need an e-ink device.
The Nook is $150 now for a wi-fi only version. Great deal.
The Nook also supports the format used by a lot of libraries to “eLend” books. Check out http://search.overdrive.com/ for your participating libraries.
I just picked up a nook yesterday. I liked the side-loading flexibility and the touch screen.