Drywall installation vertical or horizontal?
I've always been told horizontal, and it makes sense in that it makes taping easier and and reduces joints.

The main reason I've been told is that you want to avoid having a continuous seam from floor to ceiling. When the house settles, these are more likely to form a crack.

Also you don't want to place drywall seams at the corners of doors or windows. Previous owners (or whoever) put seams at the bottom of my windows and I've got nice cracks from the corner of the windows to the floor. As long as the horizontal seams are screwed very close to the edge at each student, I'm not sure how a horizontal crack could form.