No idea what's best unless we had a picture of what your doing.
If you can do it vertical in two sheets then two sheets horizontal will do the samething but you will not have to be going up and down the ladder to finish the seam.
Both ways will work just one is more work.
theres always big wars in the drywall section about hanging drywall vertical or horizontal. i believe there is no right or wrong way but whatever way works out the best then do that. so if vertical works out best then vertical is the way to go.