Which has more energy? 30J 12mm or 20J 18mm? GentelYAG.
It’s hard to say because one penetrates deeper. It depends on the area and the hair. Also, there is a pulse duration setting that matters a lot, which you’re not providing.
What you need to figure out is what works for YOU personally best. And that can be tested. Generally, large spot size, high joules and low pulse create “high settings”. So you can try variations and see how high you can go without burning your skin.