Horizontal / Vertical is mainly determined by the space you have for mounting your water heater. For example, if you have a bathroom with high ceiling and you want to mount your water heater above your bathroom's false ceiling, in this case you should select a horizontal heater for mounting so that your false ceiling can be higher. On the reverse, if you had chose a vertical heater, your false ceiling may have to be lower due to the vertical heater that take up more height space.