Kindle Paperwhite or Kobo Glo
I'd go with the Kindle Paperwhite IF you think you will be buying most of your ebooks through Amazon.
If not, go with the Kobo Glo instead of the Nook Simple Touch.
The Paperwhite and Glo have the same updated, whiter, 1024x728 e-ink screen. The Nook Simple Touch has the older, grayer, 800x600 resolution version. All have similar non-backlit lighting. They all retail for $119 - right in your budget.
The Kobo Glo supports Epub and mobi popular formats. With the Paperwhite you are stuck with Amazon's.
The Glo is also a bit lighter than the Paperwhite, but this shouldn't be a big deal.
Here's a quick comparison
of the Nook Simple Touch, Kobo Glo, and Kindle Paperwhite