Re: vertical drywall?
As others have said, the primary consideration of horizontal vs. vertical is the total length of joints that must be taped. In your situation, go vertical.
Another consideration is to reduce the amount of overhead taping. By going horizontal, you don't have as many joints to do above the 4' line, where you'll need to spend more time on ladders or stilts.
A third consideration is to make the joints as invisible as possible. Sometimes this means carefully planning your layout to avoid shadows from lighting that's near the wall, and if aesthetics trump cost, it may mean more joints to tape.
On larger walls, it's common for pros to use 12' long sheets hung horizontally -- there again, it's to reduce the amount of taping. Whether an 8' or a 12' sheet, when hung horizontally the butt joints (upper sheet vs. lower sheet) are staggered.
Your FIL might think that you have to go horizontal because he probably saw a professional do that... the pro's reason was to reduce the amount of taping. I'm sure the pro would hang it vertical given the same situation as yours.
P.S.-- I'm not a pro. I don't really like doing sheetrock. In fact, I've never been satisfied with any joint I've taped. I've just spent enough time around construction to have observed trade practices.
The "Senior Member" designation under my name doesn't mean I know a lot, it just means I talk a lot.I've been a DIYer since I was 12 (thanks, Dad!). I have read several books on various home improvement topics. I do not have any current code books I can refer to. I was an apprentice plumber for two years.