Also, everybody will listen to a Mayday and will not interrupt the ATC when they start helping. That is why I think it is better to say it (or pan pan if it suits better).
If you just state that you have an emergency in plain language some pilots in the frequency could miss that and make untimely calls.
Anyhow, a crew in distress will do what they have to do, then let know the ATC what they are doing and what are their short term intentions. And this, only after they have controlled the flight path and speed.
Then they can isolate the problem, make a decision and finally contact ATC again and state what are their further intentions and say what they need from the ATC.
It can be very helpful to sqwak emergency if you don't have time to speak with the ATC because they will take care of separating traffics on your way. If you don't know what to say, sqwak 7700.
Although now you have TCAS if they are not aware of the problem and you are deviating from the flight plan and or level. But don't forget to put it at TA ONLY if you are unable to climb.
But this is another thread: when to sqwak 7700?