i don't have any experience with a nook - but i have had a kindle for about 3 years. i really love it - i am on amazon all.the.time, so for me it makes sense to have an amazon product. i use the kindle app on my iphone and ipad now a ton (even more than i use the actual kindle) - and i've basically given the kindle to DH
i basically just LOVE having an eeader ... it's so incredibly convenient! it makes reading while breastfeeding and/or holding a sleeping baby so easy b/c you can do it one handed
eta: i think it was when juliette was a baby that i started using my iphone to read more than the actual kindle - b/c it is backlit and i could read in the dark! w/ the kindle it's like an actual book - so you have to have a clip-on light to read at night ... not sure if the nook is the same way. also not sure if you have an iphone/ipad? if so - the kindle app is great - i assume there is something similar for the nook? if not - that would definitely sway me over to a kindle ...