Generally, I prefer magazines in print. However, the more I DM the more I need a high quality digital format. I prefer PDF over print for Dragon and Dungeon magazine because they're meant to be played. Many of the articles can be used as a game supplement. However, in the printed version there's a considerable amount of crap that isn't related to game play such as advertisements. I don't want to lug around advertisements. With the PDF I print the few pages I need in the game, as opposed to lugging around and entire magazine because it has the 5 pages I need.
Furthermore, as a DM if there's a handout I can print it off in high quality rather than photo copy it or hand them a magazine and say, "alright, give it back. You're creasing the edges!" Dragon Magazine is good for monster articles. I've been printing off monster articles and creating a 2e binder of monsters for my 4e campaign. Since I don't have access to a photo copier, it's much easier to print monsters.
PDF allows me to choose what I carry as paper and what I store as digital. The best part is when I no longer need a monster or article I throw away the paper and the digital copy is always within arm's reach. Shelf space = 0