In my experience, though it's dependent on the pitch of the domed tracking wheel, is that the vertical hinge offers finer adjustment than a horizontal pivot. If the horizontal pivot has a pivot point that's far removed from the axis of the wheel, increasing the radius it's being swung, it will become finer, but if you pivot it directly under the wheel, it's quite coarse in it's adjustment.
The first grinder I hacked together had a horizontal pivot, because like you say, it was easier to make (to a point, you also need a way to tension that pivot joint, which can increase the complexity of the mechanism), but the second one that I tried to "build right" has a vertical hinge.
The only problem I have with the vertical hinge, is that if you also control belt tension with the tracking wheel, and you do this through a pivoting arm and spring, you need to be careful that you don't allow the axis of the vertical hinge to get too far out of parallel with the general direction of the belt. IE too far out of the pivoting range and your vertical hinge becomes more like a horizontal pivot in orientation, and the adjustments become less effective. Too far and the belt won't hold it in position. It's easily resolved by moving your tool arm in or out depending on which way you need to go, but something to keep in mind.
