I have always placed my heaters horizontal... reason being simply because heat rises, whether it be air or water.
Almost every single heater that I know of has the temp sensor/thermostat located at the top of the tube. There are those that have separate probes but everyone knows the heaters I am talking about.
If the heater is vertical the heated water raises up the length of it and the warmer water is more concentrated, thus standing a good chance of tripping the thermostat off prematurely..(before desired water temp is reached).
If the heater is horizontal the temp sensor is reading more of the water temp in the tank as opposed to the heated water rising off the element. Much more accurate and efficient in my opinion.
(Did all that make sense or did I do a ka-ka job of explaining it??....lol)