Well, I'm a controller and would say go with "Souls on Board" if you are carrying a dead person.
SAR/Medevac tend to have booked out with/talked to us anyway on the way to the rescue so we know the previous Persons on Board - Normally 4 or 5 anyway - So on the return leg if you report 5 "Persons On Board" we know you have a live casualty, if you report 5 "Souls On Board" we know that the're is a body on board.
Thats how we used to work it anyway. Though the crew normally make it clear if the person is living or dead - ie requests for assistance on arrival etc. but without spelling it out to the world.
If there were any confusion then we would ask them to clarify. It is very important for subsequent SAR if required.