Keep the sqk unless told otherwise. However, if you have not alraedy been identified by ATC, 7700 is a good start. 7700 is always the right sqk if you freecall D&D on 121.5
PAN vs MAYDAY. You will probably get the same response from ATC regardless (2 emergencies at the same time may muddy the waters). However, in simplistic terms MAYDAY mean that you honestly think you're about to die; PAN means you think it may take a little longer.