All in the eye of the beholder on what looks good.
If you want the ceiling to look look lower then run it horz. taller you run it vertical. I think running it vertical makes it look like slat board, but that's just me.
Paneling is faster and cheaper but will give you a cheaper look like a mobile home and should not be run horazontal unless you have some plans to add batten strips for the seams.
If you use real wood and run it vertical you going to have to add some horz. stips of wood to attach the wood to.
What ever way you run it do not try and place it tight to the ceiling or floor, there's never perfect. The base board and some form of crown moulding will cover the gaps.
Prefinish the wood before installing, It's going to expand and contract and where the T&G come together will open up and expose any unfinished area.
It would be best to install this strating in the center of the wall, that way there's equal cuts on the ends.
I use a narrow crown pneumatic staple gun when install this stuff.
Try to buy it from a real lumber yard there far more likly to carry or can get longer , straighter, less flawed pieces.
You do not want to have any cut seams in the middle of the wall.