It's good to remember that the Nook will work with ePubs from any store except Apple. Go to
www.inkmesh.com for a quick and easy way to look for authors and titles across online bookstores.
With a tiny bit of effort, you can also strip the DRM and convert a book from any of the stores, except Apple, into a format that will work on your reader. I'd be more focused on which reader is best for your needs