I always place "submersible" heaters horizontally in sumps.
And because the thermostat in the heater is usually above the heating element, and because heat rises, I believe they probably are more efficient in a horizontal option where they are not constantly flipping on and off.
Same. Horizontal is my preference. The only time I'd use vertical is when:
1) I'm certain that the water level will never go that low
2) water will constantly flow all across the heater
That might be true in a sump, but one would have to design the sump to be sure of that. It's never been true in any of my tanks, because I have always liked large water changes and because the placement of wood and rocks can sometimes restrict water flow.
Placing the heater low and horizontal in the sump would be my preference.