I've found I prefer to read PDFs on a full-size retina iPad compared to a computer.. The page size is almost close enough to full size, and the pixels are dense enough that it looks really good. But that's really best for linear reading.
When running a game, I'm usually flicking back and forth between different pages. This is more difficult on a tablet. On a paper book, you just fan the pages quickly until you get in the neighborhood, and then you flick a couple more to get to the right page. I find paper is much better at finding things when I remember "Well, it was in about the middle, no before combat, there it is."
Digital could be better if you could search for things well... but we aren't really there yet. I find it much easier to use search on SRD-based games, because searching an SRD is much easier than searching an undifferentiated PDF. You have one place you're trying to get to as fast as possible.
On the other hand, I find prepping much easier with PDFs than paper. When I'm prepping, I'm much more likely to ask questions like "Where are all the places Duke Rufus is mentioned?" I have the time when prepping to read all the results and synthesize a motivation, or reaction or plot. I don't have that time when playing, and that's when I just want a quick rules answer.
For me, the best route has been to have both paper and digital versions of stuff, so I can use the right versions in the right times.
