I have both the Nook Simple Touch Glow and the Kindle Paperwhite. As far as I can tell they are exactly equivalent in terms of the competitive niche. I much prefer reading on the Kindle. It's smaller, litter, slimmer, and the lighting is more agreeable. The nook is oddly thick and the buttons are all much too hard to push, at least when it's new (as mine is).
The nook's lighting is more uniform but the light sources are too close to the edge of the screen, which means the glare from the source bugs me while reading. The brightness controls on the kindle allow for finer adjustment and the minimum light level is lower.
The kindle's almost complete lack of buttons appeals to me, since I'm already used to reading on tablet and phone touch screens. Nook's two different power/home buttons make no real sense to me, and the page turn buttons go ignored in favor of swiping or tapping on the touch screen.
Both screens are very pleasant to look at when the light is enabled, but kind of oddly colored when it's off. The kindle's higher pixel count is noticeable, but not so much better that I'd ignore the nook. Neither screen is very quick to respond to touches or page turns. The kindle is a bit faster than the nook, most of the time.
Shopping, buying, downloading, etc is a bit easier on the nook, in my opinion. Both interfaces are more than good enough though.
The really big differences show up in the infrastructure surrounding the gadgets.
B&N's web site is, in my opinion, horrifically bad. I hate everything about it. Buying items fails frequently, for no apparent reason. I never even look at their site anymore. If I want to buy a B&N ebook, I find it via http://www.goodreads.com/, http://inkmesh.com/, or by showrooming on Amazon's site, then buy it on the nook itself.
Amazon's site is better. Searching is limited and imprecise, compared to real search engines like Google. The number of items on screen is fixed and too few, but I can live with that.
The deciding factor, for me, is how many restriction Amazon puts on the kindle. Their format is a proprietary version of the old Mobipocket "standard" with their own layer of DRM. Nook uses ePub with Adobe DRM. Both DRM schemes are easily removed, but after removal, Nook books leave you with a wonderfully useful ePub, where kindle books are still in a (somewhat) proprietary format. If I want to load an ePub on my kindle, I have to convert it first. If I want to load a kindle book on almost any other reader, I have to convert it first. Conversion isn't hard, using Calibre, but I have noticed that layout and formatting is never quite right after conversion.
I'd love to read more in the Kindle Paperwhite, but Amazon has crippled it too much to be of use to me. I don't like the physical experience of reading on the Nook Simple Touch Glow... it's just too chunky and clunky. Ultimately, I choose to keep reading mostly on my Android tablets. I buy my ebooks from places that sell them in ePub and read them on devices that support ePub.
