Look at the definition of each emergency and you will know exactly which one to use.
1.
Distress covers
aircraft threatened[/COLOR] by grave and imminent danger and in need of immediate assistance.
Anything wrong with your plane..use this one
2.
Urgency identifies a
very urgent message concerning the
safety of a ship, aircraft or other vehicle, or
some person on board or within sight.
Anything to do with pax, crew or seeing something strange on the ground (ship sinking).... use this one.
Hope this helps.
Regards RE