None of them are designed with ergonomics in mind. The Nook feels more like a book to me. The Kindle is way too skinny and a bit like holding a piece of flimsy shale tile. However, if you purchase a good solid cover for it, it feels much better. I own the Kindle-3 but often wish I would have splurged on the color Nook since it has a back-light.
Here's a good
rundown of the Kindle-3 that has a photo showing several e-readers side by side.
And here's a
review of the Kindle's lighted cover which I wish I'd never bought. It'll work in a pinch, butit really doesn't put out enough light to suit me.
I love the feel of the Nook, but I sure hate touch screens. All e-readers leave something to be desired though.