Mechanics are more important, and studies have shown the a FB puts more stress on the elbow than a curve. To me, what’s more important that the kids keeps their arm in throwing condition and works to keep the muscles, tendons, and ligaments strong. That means doing doing a Thrower’s Ten type of regimen, and throw! That doesn’t mean year-round pitching. It means to throw some year-round with some periods of lighter loads to allow some recovery, and keeping an eye on innings where the kid struggles. If a kid can’t keep an inning to 20-25 innings, they need to sit and wait for another day.