Assuming that the quality of both sources is the same, and you do have a relevant book near at hand, the book will always be the obvious choice. There isn't a scientific study anywhere that says you can find reference information faster on a computer than a book.
But you don't need to be a scientist to prove that to yourself. Put a dictionary in front of you and track the time it takes to look up "sesquipedalian". I can guarantee that you'll find the definition faster in your paper dictionary than it'll take you to even find an online dictionary. You'll probably find it faster than you can even type in the word on your computer!
(The word means "using excessively long words", by the way.)
The only counter-argument I can think of is that you might not have a relevant book. But that's a different question. That would be "computer vs. no books". I think we can all figure that one out!