pre 52 alan scott is much more powerful, considering he has a wider range of abilities that can affect things on a wider scale. Superman is very powerful, but only physically slow. hes essentially strong, fast, and hard to hurt; were alan is a sentient manifestation of mystical willpower in the universe. new 52 universe doesn't have the feats to know for sure
Being immune to super strength isnt seen much. And there is rarely such a thing as true power immunity. But someones power would have to be more powerful than the immunity to be effective. For example human torch is immune to fire but only up until a certain amount.