Depends entirely on what I am doing.
Overall--Books. I can flip to other chapters without going through a scroll of things, and my spatial memory serves me better with physical books. If they get wet, they function. If the stewardess on the plane tells you ALL electronics have to be off despite showing her it is in airplane mode, a book will rescue the day. They're cheap, and there is no such thing as a half-price-books for e-books.. and their batteries never die, and you can write cool stuff in them like "Remember this paragraph, it is useful later" to yourself, and shove stuff in them like notes, bills you were suppose to pay, money, and pictures. And your textbooks never expire. My electronic textbook 'expires' when my class does (one year from now) but if I keep my physical copy it never goes away. I'm that weirdo that actually re-reads textbooks.
However, e-readers have bridged a gap that books sorely lack.
They have samples. You can download a sample from the store, read it later, and decide if that book is something you want or not. You can buy more books without having to wait until you get to a store. They travel better--you can take an entire collection and series with you, not just one or two because of weight issues. They have LIGHTS. You need nothing else to read that book except the platform itself, and usually they can be dimmed so as to not disturb others. You can listen to music while reading them on some. You can have a book everywhere you go--suddenly stuck in a long line? Your smart phone has your e-book app right there. You can bookmark page numbers and instantly see that archive. You can word search--probably the most useful part of the whole thing. You can search hundreds of pages for that exact bit of text you want to read again. You can highlight without the need of a highlighter.
(They still have further to go. I'd like to see it where when you bought an e-book you could download the audible version with it as well, so you can choose to read it or listen to it, or both at the same time.)
I prefer both, but I like the idea and concept of books more. They help me more when studying than e-readers, and I get more enjoyment out of seeing how many pages I breezed through when I should have been sleeping.