device wise I much prefer the Kindle over the nook. I have a pre-order in for the new Kindle 3 and I'm just on pins and needles to get it into my hands.
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nook
advantage - Overdrive Library books
advantage - Can shop from barnes and noble, borders, kobo, books on board etc
advantage - there's some other things people list here like the replaceable battery and the memory card, however the reason
Amazon discontinued these from their models was that they found the reduced case integrity was causing malfunctions. This seems to be the case with nooks too.
disadvantage - that touch screen. I hate it. Hate it. Hate it Hate it and I just can't make it work right. I know that other people list that little thing as an advantage but I've just decided to come to terms with my dislike of that little bugger and state it proudly.
disadvantage - If a book isn't Agency priced, Amazon is probably selling it for cheaper
disadvantage - no International wireless access
Kindle
advantage - newest battery is supposed to last up to 1 month w/out a charge.
advantage - low low low low low book prices
advantage - huge catalog of book availability
advantage - best screen (new eInk pearl). I saw this screen about a week ago and it is far better than my K2 or the nook.
advantage - International Whispernet (ie you can buy and download books when you travel out of the US)
advantage - of all the eReaders this is the one that ergonomically I like the best. Great button placement, ease of use etc. This is largely a personal preference, you'll need to check them out yourself to see what you think.
advantage - 30 day no questions asked return on Kindle itself and 7 day return policy on eBooks (ie if you click buy on a book on accident Amazon will refund your money).
disadvantage - no Overdrive Library books unless you learn how to mess around w/ the Python scripts. This isn't exactly hard to do but it is a pain to learn and it also runs up against all those DCMA laws.