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Replacement heads are always preferable but not always convenient to find. Personally, I use the save-a-thread inserts over the helicoils for plug holes. The reason I do is that they are a bit more rugged in my opinion then helicoils.
I have observed that helicoils tend to allow cross-threading to happen again pretty easily. Also if they’re installed wrong (it’s kind of a finesse job to install them…only experience makes you good at it, and you generally get 1 try otherwise it’s ruined anyway), pieces can break off into the cylinder. I had this happen personally, where a piece of the helicoil broke off, stuck to the piston, and lit up like a christmas tree causing pre-ignition, and actually melted off the end of the piston (this all happened in about 15 seconds at WOT, which when you’re on the boat, by the time you look back and see a cloud of smoke and steam, it’s too late to do anything i.e. slow down/kill off the motor before seizing).
Helicoils also come unscrewed with the plugs, which just means if that person tries to reinstall the plug with the helicoil already on it, then everytime they do this and snug it down, they’re screwing up the soft aluminum more and more.
The save-a-thread inserts bore out the hole, screw into that same hole after being retapped, and then you take a hammer and installer tool (included in the package with the hole tap) to fold down a upper sleeve which locks it into place. No worries about getting stuck to the plug, or ever backing out. Plus by putting some thread sealer, it stops any leakage of oil/gas and seapage of compression on the power stroke.
Whatever you do, I would not recommend following the directions for installing either of these options by applying grease to the tap. Remove the head, install the insert/helicoil, clean it all up, then put the head back on.