Most of the specs on these things are more or less the same and it comes down to personal preference.
First important question is it for you or is it a gift?
- If it's for you or someone tech savvy, then buying a top of the line reader is the best because you can get all your books for free (if you want) via piracy and eventually recoup the cost. I'd recommend the Kindle paperwhite.
- Otherwise if it's a gift for someone or they aren't good with computers: go for a Kindle 6th generation (cheap) or a Nook. Both of these are linked to the online bookstores of Amazon and Barns & Noble respectively, so buying a book for them is as easy as clicking 2 times and you're good to go.
I've been using the Kindle Paperwhite for 1.5 years and it's performed great. Now there is a newer version with 40% higher resolution and bigger ram for the same price. It's at the top of your budget but spec wise it's the best. The most important things to look for is:
1) what brand the screen is - the best are made by E-ink and their top model is currently the Carta
2) Processor speed - this will affect how fast you can turn the page and how fast the new one will render. More is better here.
If you want to save on money the biggest difference is whether or not the screen is backlit. Basically this allows you to fine tune the brightness of it depending on where you are reading. So if you are not going to use it outside or read in the dark then you might be better off with getting something cheaper (Kindle 6). However you will have to put up with slower page loading and turning.