Nook vs. Kindle .... which to choose?

I am going to get a nook/kindle for my birthday from someone and I get to pick if I want a kindle, or a nook. I'm a book worm and I don't mind if there isn't color. I want a big range of books to choose from, and I don't want something too expensive. I would like color if it were possible to get it, but it isn't my main focus at all.