They all have the same E-Ink Pearl screen with the same contrast, the same resolution, and the same type of IR touch-screen sensor. (Any screen differences in the photos are the result of uneven lighting, not any real differences.)
They’re all available for $99, but the Kindle and Kobo both show ads (“special offers”) at that price — if you’re looking for an ad-free reader, the Nook is the least expensive at $99, with the ad-free Kobo at $129 and the ad-free Kindle at $139.
Content libraries and ecosystems
All of the major e-readers have similar content libraries these days.
The Nook is particularly good for magazine availability, even slightly exceeding the Kindle’s availability in my searches. But Barnes & Noble’s content store is very buggy for me: I often get errors claiming unspecified problems with purchases, downloads, or connectivity. (This also happens on the Nook Tablet.)
Kobo’s ecosystem is still a disadvantage. In my searches, while book availability was pretty good, it had the highest prices most often. And critically, its magazine and newspaper selection is abysmal — if you intend to read magazines or newspapers on your e-reader, you shouldn’t consider Kobo.