Whichever way you look at it, the big deal with both the Glowlight and the Paperwhite is the fact that they have a light up, 6" e-ink technology display.
In fact, with the Glowlight, that's pretty much the only deal. It's full name is the Nook Simple Touch With Glowlight - which is exactly what it is. It's the Nook Simple Touch with the addition of a light.
The Paperwhite, on the other hand, is an upgrade of the Kindle Touch, which it now replaces. Yes, it does have a light, but it also has a whiter than previously seen display, more pixels per inch than either the old Kindle Touch or the Nook, and a slightly textured surface on the display.
The Paperwhite light is also more evenly distributed than the Glowlight's - thanks to Amazon's light guide technology which spreads the light from low energy LEDs uniformly over the surface of the screen. There are still some high spots at the very bottom of the display (where the LEDs live), but that shouldn't bother you too much.
It's worth pointing out that both the Glowlight and the Nook project their light onto the display surface rather than back-lighting it from behind, such as you might find on a computer or smartphone. That means that either reader still offers you the enjoyable readability of e-ink displays without eye strain.