Discussions raging elsewhere on the applicability of PAN vs. MAYDAY calls, and when the future viability of the craft is in doubt, how the Mayday should be used.
In a number instances, Co. SOPs will dictate the call - but at the end of the day, the PIC will make the decision on what to declare.
Here is a good summation from UK AAIB on the fuel emergency declared by a VIR 744 near EGKK, and the appropriateness of the Mayday use.
Personally, I would prefer to declare a Mayday and downgrade to PAN in all instances - this gets everyone on their toes and clears the way for immediate terra firma...and when the nature of the emergency is clearer and under control, donwgrade to PAN whilst still preserving priority.
Boeing 747-443, G-VROM 07-07.pdf