To anyone with a grasp of English either might be appropriate depending on the nature of the failure and performance.
Or French, since both phrases come from that language.
m'aidez = help me!
en panne = broken/failed/not functioning. "Houston, ahhh - We have a problem!"
There are, of course, single-engine failures, and single-engine failures.
An uncontained failure that sprays shrapnel through the wing and cabin and lights up the EICAS like a Christmas tree is a Mayday, even with 4 engines (Qantas A380).
Definitely commander's call. And as the saying goes - if there is doubt, there is no doubt.