I'm in the minority, but on the whole I prefer PDF for anything that isn't a core rule-book or a world-book of some description. The reason - it's easier to cut and paste. Take something like the splat-books. On the whole, I only ever need perhaps a dozen PrC from the entire collection on any given day. I allow perhaps half the spells. One or two of the items, and the occasional spare rule. On the whole, maybe a single splat's worth of space, if that.
What do I carry around? Four full lenght sourcebooks that are mostly taking up space. Considering the scattered nature of the stuff I use, and the fact I don't game at home, I'm pretty sensitive to how much I have to cart around.
The best gaming purchases I've made are things like Ambients Lieber Equitas products, which allow me to print the PrC I need and ignore the rest. Siminlarly, the various Malhavoc e-books have proved a boon because I only print out the spells and items I allow.