Bb,
I have a small bridgy clone and I purchased a few years ago a right angled head that converted the machine into a horizontal.
I reckon that the difference in usage has been 99%vertical compared to 1% (maximum) horizontal, and that wasn't caused by the setup times. It just allowed me to do certain jobs quicker, but with more time it could just have been easily done by the vertical mill.
I have the same view as Charles above. At one time, horizontal mills were really all you had, in fact my first learning course was using one, but nowadays, I personally would suggest just stick with a vertical one, and if possible, get a right angle head for it to allow you to do those long drillings and boring jobs on the mill.
On the other hand, as already mentioned, with the correct shaped (very expensive) tooling and precision spacers large intricate cuts can be done in one pass, plus if you can gang them together, difficult jobs can be done with ease, like finning
John