I have worked a ton of medical emergencies, every time the pilot has keyed up saying "we are declaring a medical emergency, we have a passenger with xxx problem". I have never heard a commercial jet call and declare themselves lifeguard, have had where they depart in that status obviously carrying some organ or patient for transplant etc. From my view if I was the passenger on board that is in medical need, I would rather you declare an emergency then go to lifeguard status. Lifeguards I try to go direct with, emergencies go direct regardless, both get no undue delay, but emergencies get the attention of everyone, lifeguards are routine.