I have whatever the newest Kindle is. The eink screens they use on the Kindle and the black and white Nook is much easier on the eyes than the LCD used on the Nook Color and iPad. I work on a computer all night at work and take multiple online classes. I don't like reading lots of text on LCD screens, it tires and dries out my eyes. With the eink you can read them in direct sunlight, but you also need an external light to read them in the dark since they are not backlit.
As a reading device it works great. Books can be quickly downloaded via WiFi or transferred from the computer. With wireless off my battery usually lasts a month before needing a charge. You can start reading on the Kindle and pick up where you left on on a computer or smartphone. If you're trying to pick between the two I would just look at the books available in both stores to see if one store is missing any authors you enjoy.
You can find lots of free books online. Each vendor's online store has free books, including many classics since they are in the public domain. Be sure to read reviews as there can be multiple versions of free books with varying levels of quality of formatting. Project Gutenberg also has a ton of free books.
I think the Kindle is great and am glad I have mine. Where I used to take a laptop for short trips, now I take the Kindle and my iPhone. I do all my reading on my Kindle and everything else on the phone.