E-Book
1. They don't occupy physical space, which is a problem lately.
2. They don't "fall apart" with usage.
3. They're obviously as light as the device.
4. There are books that are hard to find, most of them can be easily found in digital format. Here in Brazil we have a huge lack of fantasy and sci-fi (unless it's a screaming hit like Lord of the Rings and Dune, but you won't find Forgotten Realms for instance), so the digital format got me way closer to those books.
5. Most e-readers have a nice interactivity with the book that might come in handy, such as X-Ray (Kindle, it picks important stuff and search throughout the book and at a kindle thingy that gives info on characters, universe, etc), and a direct path to wikipedia, etc. I know, you can read your book per se and just search the internet, but it's nice to have it all on your fingertips.
6. It's usually cheaper
HOWEVER, don't ever underestimate the book itself. Recently I've been looking for Musashi by Eiji Yoshikawa, a supreme masterpiece, couldn't find it on Apple's bookstore nor Amazon's Kindle Store, so I bought (for some heavy 100 dollars) the boxset containing 3 volumes (the way it's been divided here in Brazil).
I PREFER e-books, but if a certain book I want is not available in digital form I'm not going out of my way to find it that way, I'm just gonna pick it up at any bookstore and read it. Plus if I really LOVE a given book, and there's a special edition for it, I might take it, just for the sake of it.