I don't own a Kindle or a Nook, but it's not true that you can ONLY buy ebooks from Amazon. You just definitely CAN'T buy books from Barnes and Noble or the Sony store. I mean you could buy them, but they won't display on your Kindle.
Any book that doesn't have Digital Rights Management can be converted with the Calibre program to a format the Kindle can read. If you choose a Nook, then you're not going to be able to get books from Amazon or Sony. If you had a Sony, like me, then you can't buy books from B&N or Amazon.
Amazon arguably has the best selection and prices, especially if you mostly want to read new bestsellers. Don't start kicking yourself just yet.