All charged attacks work at quadrauple damage when fully charged so Fist OF God goes up to 160 damage, whilst Invincible Fist reaches 400.
Fist Of God is the better offensive technique due to its shorter charge time and forward movement, and Invincible Fist works well on the defense as it will launch any opponent (except the final boss) and due to its long charge time you can start charging right after launching someone, and by the time they get up and move back towards you can unleash it again.
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Invincible Fist is an Azel–killer. Hit him with it, backflip once, charge it up again and release it when he stops his run towards you (he runs, then stops briefly, let go of the Invincible Fist at that point). It’s totally totally spammable and therefore silly.
Fist of God is awesome as part of a combo string and I love its forward movement.