Under most circumstances I would say print, for the artwork, especially. And also because it does help support the companies we want to continue to flourish and put out more books. (PDF format is good for these companies too, because it's cheap for them, and a great way to sell more to those who normally would not be able to drop 20-30 bucks per book).
But I voted for depends on what it is. MojoGM and I were actually discussing this the other day. PDF is a great alternative for those of us who can't afford to buy every book we want in print form. What we had talked about doing was getting the PDF, printing them out and having them done up at Kinko's with a spiral biding and possibly laminated. That way, we can have a lot more books until we can afford to splurge on every print form book we want.
And it also depends on how much you are going to use something. If you see something in PDF that might be great for a short while but that you might not need to go back to a year from now, PDF is a great way to do so.
Working in publishing, I can say that I think all books look better in print form. But when money is tight and information is more important at the moment than a nice book on your shelf, PDF, by all means.