I like reading on an iPad, to which you can download the free Kindle Reader and happily buy their e-books or download the Kindle freebies (mostly older classics, lots of good Thomas Hardy on there). Then you can go visit the Apple book store - which has a number of free downloads too - so you get both. I also like that the iPad is backlit, though the newest Kindles may also be so now, not sure what the latest ones are like.

But I share EBarnacle's feelings about Flash memory, and there are a number of limitations to all mobile devices that mean I still use my laptop for my actual work (when I manage to do any). I wouldn't keep anything sensitive or overly valuable on the iPad, but boy is it fun for reading, playing with the apps, and generally time-wasting. So, because you can do so much with it, that's what would get my vote. Have to admit, though, I haven't seen the Nook so am only comparing the two readers I know.