Basically the smartest thing would be taking the Demon's King offer.
Kill all, go to floor 99, try to not get killed by the ultra mega super powerful mobs and get killed by the final boss to get some super even more cheat-like ability. Restart and take the Goddess offer.
But i don't think he's actually that evil to just kill all (who just acknowledged his worth), so i guess he'll take the Goddess one.
The main problem is the fact that he'll restart the interrogation, where he will need to admit to have killed innocent people (the rankers), so it's pretty fucked up.
Let's see tho.