Tablets are heavier. They're designed for games and video rather than reading. The Kindle Fire has battery life of about five hours in my experience. I love my Kindle Fire but it weighs a ton so I can't read books on it. It's like a cheaper iPad basically.

An e-reader is like having a book that happens to be electronic. They have a different kind of display you can look at for much longer, are much lighter and the Kindle ones have a battery that goes for weeks or even months.