In the heat of the moment nobody in ATC would delay in responding to any such call, regardless of the call's prefix.
What might be worthwhile discussing is that controllers tend to 'tune' their ears to the words MAYDAY or PAN so that in the unlucky event that an R/T transmission crossed with another, they may well be able to pick out that word and respond slightly quicker.
If an engine fire breaks out on an occupied aircraft, would you not call MAYDAY instead of PAN?