Probably you'll be either as happy or unhappy with either choice as you want to be.
I have a Kindle 3 and a Toshiba e800 and I use both, and I'm going to look for a thread on which is the best ereader for library books to check books out, but it will need to be a cheapie or DH will cut up my credit cards.
Two questions to help you decide:
1. How sensitive are your eyes to reading a backlit screen? I see tons of posts on the Kindle board about people whose headaches and eyestrain were relieved by getting a Kindle. Personally, I can read on my laptop all day and night without problems, but I seem to be in a minority. If you have problems with backlighting, definitely go for the Kindle.
2. How important is getting library books on the ereader to you? Do you have access to a library with lots of e-titles? I still check out lots of paper books because I can't see buying books I'm probably only going to read once, and I have over 250 unread books on my Kindle, most of them free from Amazon. But if you expect library use to be a major portion of your e-reading, get the Nook.