My 02 cents
If you have an emergency and you have done the initial sorting of the aircraft the first words on the radio should be Mayday,Mayday,Mayday. This tells ATC and everyone else on the frequency that you have a problem.
If you are in doubt as to whether you should call Pan or Mayday, then there is no doubt call Mayday, you can always downgrade it later.
Anyone who implies that you are a lesser pilot because you called a Mayday when maybe you did not have to, is a stupid fool and should be completely ignored.
4 Mayday's for me so far.
Engine failure on departure in a C 150
Engine failure on departure in a PA 31
Control problem due to misrigged trim tab in a T 28 Trojan
Major electrical malfunction which resulted in a partial panel battery power only ILS in a C 421