The data I've seen seems to show that the Moderna vaccine is the best of them in terms of long-term efficacy against breakthrough infection and hospitalization. I got Pfizer, and honestly I wish I would've had Moderna regardless of some very slim chances of myocarditis. My increased chances of getting a breakthrough infection on the Pfizer vaccine outweigh my chances of getting myocarditis on Moderna.

Viral myocarditis is serious but not life threatening and self-resolves in less than a year; the vaccine-induced cases are even more mild than that.

That's very unclear, since people who got Moderna are getting fewer breakthrough infections that people who got Pfizer, and are being hospitalized less (from studies by the Mayo Clinic and confirmed elsewhere). So you can't look just at side effects, you also need to look at effectiveness. They could inject you with a placebo known to be side effect free.

Is it likely that if both the vaccine and the virus cause it, that the symptom could be related to the spike protein itself?

In that case, I'd expect all of the vaccines to have a similar risk, and it's only been spotted in one so far

It has been clearly spotted in 2 mRNA vaccines (Pfizer, Moderna). There is limited information about vector based vaccines.

It is fairly clear that adverse effect is dose dependent and it may be the reason why we do not see that many with vector vaccines - immune system creates some immunity against the vector virus and actual amount of vaccine entering cells is limited. It is also evident from the limited immune response from vector vaccines.

The best available data suggests that _that_ particular shot is rather more effective after a few months than its competitors.