A non-holding back Superman punches harder. Punching through dimensions under his own power is saying something, as well as casually punching a 1600lb being through two harder than steel thick walls AFTER REACHING SPACE.
But Thor is not far behind. Especially knowing that the hammer cheats for Thor and supplies extra strength to Thor's own when Thor is swinging it. This is because the hammer can move on its own as if it is living, all based on Thor's command of it.
Also one must know that for Thor to hit really really hard he must hurl the hammer faster and faster before delivering the hit. This is like someone receiving a running start. Don't let Superman get a running start.