(I'm not aware of the current situation in the US btw, just know that it's perfectly possible to get a booster in the NL)
But now that the data is in, though, when your option is two vaccines, one with very minor risk, and one with even less risk, and they are both equally effective, it's obvious that you should prefer the latter.
https://www.fda.gov/news-events/press-announcements/coronavi...
1: The Moderna vaccine is separate from Pfizer/Biontech even if they share technological base.
2: Moderna was discontinued for younger people (below 30yos) in at least some European countries due to slightly higher risk of myocarditis (even if the Pfizer vaccine had slightly lower efficiancy).
Seems like both vaccines are allowed for smaller children in the US now. Over here they put the cutoff for the vaccine at 12-13yos iirc (my older kid got it but not my younger one).