Captain. I have done over a decade of testing on which frequency a pilot is more likely to get an instant answer on,
From my experience 121.5 winds hands down- especially at low level.
On 121.5 there is a big difference between:
- doing a check call, at altitude in a modern well equipped aircraft v.s.
- calling MAYDAY in a bug smasher suffering an emergency with hands full on the way down ...