When I did my house, I ordered 250 4'x12' sheets that I and some friends installed horizontally. For that application, it worked well and the tapers were happy. The only thing I didn't like was the vertical joints where the 4' ends butted together isn't tapered and left bulges you could see when looking down a long hallway or room wall. But with horizontal, you also tend to use less drywall on anything other than 8' walls.
I just finished the drywall on a 15'x15' room and installed 4'x8' sheets vertically. Since I installed all the firring and I knew they were 16" O.C., there wasn't a problem and I didn't have that issue with butting the 4' ends. And since I'm taping and installing the baseboard, vertical was the best option for me.
The tapers created on the long edges of drywall make taping so much easier. I like to take advantage of that. Horizontal installations on an 8' wall leave one taper right where the baseboard is installed, where wall and floor meet. That can create problems when installing the baseboard as it tends to be sucked in at the bottom (if you nail it there) leaving a gap at the top.
In the end, it's a personal decision, but for me the specific application is what determines vertical or horizontal.