I consider this kind of like comparing apples and oranges. That is to say, they can both be great, but your reasons for enjoying an apple are going to be quite different from enjoying an orange*.
As with dice, there's something about a physical product that the reality of digital books simply can't replicate. The advent of eReaders and digital books has changed a lot about the industry, but there's no replacing the feel of a physical book.
|Hey guys, check out my book collection!|
Digital copies are often at a similar price point as paperbacks, and they're very useful in their own way, but there's one particular difficulty I've found with them: Loaning. If you want someone to check out a game system, or even just to look something up for you, it's really simple to pass a book to him. If all you have is a PDF copy, you can still pass it on, but that's . . . not entirely legal.
So, what do I prefer? It's rather a moot point; I buy so few RPG books generally that it's usually just a question of what product they've actually made available.
* - On an unrelated note, someone should really create a word that rhymes with orange for the sole purpose of having a word that rhymes with orange.