Does it make a difference. My Exmark has a horizontal and my Super Z uses a vertical any reason why one is better than the other? P.S. I've never replaced the mule belt (Exmark 900hrs) so that's not a reason for me.
Not many companies use a horizontal shaft and the mule belt is the reason. Exmark uses it because they wanted to be able use engines that are not available as verticle shafts in their mowers. It doesn't really matter to me, it just so happens that Dixie uses a verticle shaft on my particular model.
I prefer a vert. shaft because there are less things to go wrong. It is a direct transfer of power, instead of changing direction of the force applied many times. each time you change the direction, you loose some power. you are only substituting what a vertical engine already has.
that said, there are some engines that only come in a horizontal shafter orientation, like diesels, and most liquid cooled engines.