Law's is definitely more powerful, but drains stamina way worse. If it was kidd vs law, law would have an advantage at first, but as the fight dragged on or if kids learned to use his conquers haki he'd lose.
That's true, but he can't do that for as long as kids can throw shit at him. His best bet is to take him out asap, but that goes against his nature (he'd rather save lives than take em)