I'm 6'4", and one time I borrowed a hori/verti splitter from a colleague (forgot the brand, standard yellow one from the orange store. CHampion?).
I ran it vertically, and I liked it. The crucial point is to have a helper that brings rounds to you, so you can stay seated and efficiently run the splitter handle. Throw the splits (far enough, so you don't build yourself into a valley of split mountains) away, take the round just brought to you, and go.
This was with 20-30" dia red oak, so not light. Just have to handle them like gas cycliners, tilt and roll on the bottom circumference of the round.
I ran it vertically, and I liked it. The crucial point is to have a helper that brings rounds to you, so you can stay seated and efficiently run the splitter handle. Throw the splits (far enough, so you don't build yourself into a valley of split mountains) away, take the round just brought to you, and go.
This was with 20-30" dia red oak, so not light. Just have to handle them like gas cycliners, tilt and roll on the bottom circumference of the round.