King David VS Herakles, who was stronger?
Herakles was an inhumanly strong demigod, whereas David was your standard mortal shepherd. David did not take the place of Atlas in holding up the earth; He just killed a bunch of people, including setting up a guy to die in battle so David could have his wife.
A better biblical analog of Herakles is Samson.
A better Greek analog of David is probably Odysseus.