If my colleague was incapacitated I would call a Mayday.
My reasoning for it being a Mayday is that I require immediate assistance and would like a controller to help me get on the ground quick smart. I want to be on the ground quickly so that:
a) my colleague can get immediate medical assistance and,
b) I don't want to be in the air any longer than necessary as if we were to have another problem it will be a lot more difficult to manage single crew.
I was taught that at FlightSafety, they briefed us to call it as a Mayday, and I certainly had no problem with that.
windypops