PAN and MAYDAY in flight both do the same thing. Get you all the ATC help (vectors, priority, landing airport suggestions) you want.
So why the two different calls? With PAN it's just you, the controller and a local fire callout (if fire services are available).
With MAYDAY the controllers will also activate the disaster plan (notify the hospitals, give a heads up to the Ambulance service, get the police to think about road blocks etc etc).
So yes - if priority and a bit of Nav help will solve your problem, it's a PAN.
If you think, despite your best efforts, it may all turn to tears and you and the passengers might need a hospital - it's a MAYDAY.