I've always sold more horizontal (landscape orientation) prints than vertical (portrait orientation). But, I'm more likely to sell multiple pieces (a complete set or series, for instance) when selling vertical pieces.
In my job as part-time decorator, when looking for verticals I'm more likely to look for sets of 2 or 3 pieces, then hang then side-by-side or one atop the other.
Also, I have a serious liking for creating square art. In the last few years, my squares sell about as often as verticals, and seem to be more popular all the time.
*Warning -- Unsolicited Advice Alert!
Susan, I'd consider moving the print in your picture, above, an inch or so to the right. As is, it's nearly bumping into the ladder and is not comfortably situated in the empty wall space. Feel free to ignore! :-)