Both are more powerful than Superman. Or do you think that Despero defeating Superman simultaneously with Wonder Woman and Martian Manhunter isn't enough of a testimony to his strength? And I disagree with your last statement. Even if he is stronger which I do agree with, it isn't as great a difference as you say.
It's a high showing, obviously. It's just that he's normally written to be around Supes' equal in regards to strength with his mental powers making him a much bigger threat. Take away his mental powers and extra eye and he's just Mongul with less fighting skills.