The cheaper Kindle isn't waterproof, and the display resolution is lower.
Non-waterproof means don't drop it in a puddle, not that a few raindrops are a problem.
The display resolution is fine for text, but any images won't look as good (not that they are ever terribly good with the limited greyscale).
There's also no extra-cost option for loads-a-memory (only needed for comics full of images, or for audiobooks) or for data transfer via the phone network.
I find that "weeks of battery life" equates to 4 or 5 regular paperback length books, and that duration depends more on how fast I read.