..but what are you using it for? A straight Kindle is OK, haven't used the Nook but admire the native number of formats it supports.
There are freebie conversion programs from Gutenburg to at least some of the available formats for just about any reader device you can name.
I had originally gotten the Kindle with the idea of downloading many PDF manuals into the beast (which I did) but with the standard screen version I often have to go landscape to read those manuals at all. Diagrams are often too small to be useful. I do like the device a lot but....
If I had to do it over again Maybe the Kindle DX (big screen) or more likely the iPad. The thing about the iPad is it really is a decent reader (for a device in mid 2010): between Kindle App, Apples native App, a PDF viewer, SPL Mobile, iBookshelf and eReader; an awful lot of bases are covered. The screen is clear, it has good support for zoomin/zoomout.
A downside of the iPad (mentioned by Penn of Penn and Teller) is it has so much else on it, it is very very easy to end up watching a movie or something instead of reading.