Depends on what the helicopter pilot is doing?! Is he flying tours above Orlando in a JetRanger or going head to head against an Iraqi tank column in an Apache? Airline pilot vs. helicopter pilot is like comparing a job to a vehicle(apples to oranges), if you want a true answer compare job to job(airline pilot vs. police helo pilot) OR vehicle to vehicle(airplane vs. helicopter).
Another important thing to remember is that stressors are very different for each person. I never really considered any of my airline jobs all that stressful. I'm under alot more stress in my corporate job than I ever was at the airline.