I think any e-ink reader should be pretty easy on the eyes, and you can adjust the font size and font as needed as well. If you get a lit one (like the Kindle Paperwhite) it should improve contrast as well.
Beyond that, it's pretty individual, so you could look at a Kindle Paperwhite vs a Nook Glow vs a Kobo Glo at a store that sells them and see which feels best to you.
Also consider what store you'll want to buy books from. If you'll want Amazon, you'll want a Kindle. I think there's a way to support local small bookstores with a Kobo Glo though and that's nice.