I have both, but I prefer the Vertical shoulder rig. It's more concealable for me and more stable with motion.
Can't wear a "Horizontal" Rig truly horizontal, as the Muzzle End will stick out my back.
Have to adjust it so it's canted down towards the front, to bring the Muzzle end inwards.
It is a bit faster and easier to draw from, but it tends to move around more on me.