In your opinion why is the Time-Sert a better option?
I have used both with equal success, maybe there is something I need to learn about this.
I've used both also. Not really worried about either one relative to the other. HeliCoils are certainly far more common and I've never understood the distaste that some seem to have for their use. A product can't be around as long as they have and been used as widely and really be much of a problem.
Speaking specifically to the cylinder studs my experience is limited but what I can offer isn't anecdotal. It is my own experience. Even with a properly set red loctite had an incident of a coiled insert later walking after a removal then during the subsequent reinstallation. The top shoulder on the TimeSerts keep that happening. The inboard walk is a rare issue with helicoils. Not something I'd worry about on other than very critical and difficult to reach installations. But on those installs the top shoulder on the TimeSert is an added insurance.
There is (or at least used to be) a TimeSert kit specifically for Harley cylinder stud holes for cases where a coil insert install had failed. Found that once when someone else's coil insert had failed. Don't know why with any certainty. The appearance was of degradation (for whatever reason) around the coil. Coil both then couldn't stay as tight and could expand. The one piece inserts can't do that either.
It's just a couple of instances; I know. But it's a couple out of a total experience of at most six installs that I've either done or been in after first hand. HeliCoils are fine. I use them and don't worry about them. But out of an abundance of caution I'll go the other way on cylinder studs in these aluminum motors.
I don't remember the tap sizes for each type of insert off the top of my head. Have a (perhaps mistaken?) recollection that the tap to install the TimeSert insert is just slightly smaller than the tap for the coil inserts also. If that recollection is correct that would also suggest another small reason to lead one way or the other.