There are antenna computer programs that predict antenna gain and pattern, with both horizontal and vertical yagi antennas.
I haven't looked at them and found out what they claim.
Vertical will be parallel with a mast and tower, where horizontal would be completely above those.
Hygain used to claim that vertical had more gain the horizontal.
That may be, because the way one might have more gain is by lowering the angle of radiation and maybe a vertical does have a lower angle.
In theory antennas should be at least one wave length above the ground to minimize ground loss, but a half wave up is considered sufficient for use.
A full wave up would be around 35 feet to the bottom of the antenna elements, horizontal or vertical, but vertical would require another 9 or 10 feet of height to the boom, even for a half wave length height od around 20 feet to the bottom of the antenna elements.
So 30 feet up minimum to the boom for a vertical and only 20 feet up for a horizontal.
I had a few Jo Gunn antennas some years ago.
The screws mine are with were no stainless steel and would rust.
Maybe try to find a source for stainless steel screws.
The other thing was some screw holes were drilled the proper size and the screws tightened to the tubes without problems, but some screw holes were drilled to small in diameter, or depth and without drilling them to the larger, correct size, I broke a couple of the screws trying to tighten them down.