A Heli-coil is really only advisable for a one-time fixed repair. This means the fastener is basically permanent, as opposed to being removed on a regular basis for maintenance or even future repair. I use time-serts exclusively for all thread repairs. This is Ford's recommendation for spark plug thread repair. There is a special kit for this repair.
Ford has had an issue on their Triton motors with the spark plugs blowing out of the head. This is from repeated removal/thread damage/air-tools/over-torque etc..(I think all aluminum threads with a steel fastener face this outcome eventually.) The repair is professional to say the least. They use an undersized threaded bushing that is seated, and pressed in place...virtually eliminating the possibility of the thread coming out when removing the fastener! http://www.timesert.com/