For vertical, picture something like this:
For resale value, I think vertical is the way to go (we don't plan on moving any time soon, but we both think you should always keep future buyers in mind)
For horizontal shiplap, picture something like this:
Here are the built ins now. I can picture both but I would say vertical may look best BUT horizontal could look great as well.
So what are your thoughts? Do you prefer vertical or horizontal?