I have the plain Kindle (not the paperwhite or fire) and find it easy to read in bright sunlight, and in bright indoor light – but more difficult to read in softer indoor light. I have a book light that came with the cover and find that helps, but it also produces glare on the screen, which is totally annoying. So I'm considering getting a paperwhite.
If you want a "mini" tablet, I'd suggest the fire, but if you don't want a reader that surfs the web, checks mail, etc., then consider the paperwhite.
I can't speak for the Nook, or other ereaders as I've never used them.
PS: Moving this to the Tech forum where I think it will get more response.