I have built and used both types for large rounds.
The Vertical splitter was more dangerous.
The hydraulics pushed the cutter/splitter into the whole round, and sometimes would be 3/4 thru before the split took place,,,,violently!
The horizontal type was smoother, and always had better control because it did not have that 90 degree "hinge" to make it looser.
Loved the ones with the higher 2 stage hydraulics that "kicked" in when the going got tough.
Also, the horizontal was much easier to tow around,,,more compact.
And, really no problem rolling up rounds onto the horizontal beam.
I burned approx 12 full cords of wood a year!