That's one of the things that has so intrigued me about Jerry Siegel's creations and how they've morphed and evolved over the years. Superman has become a very 'New Testament' kind of character, exemplifying the hope that might could be used for right, that compassion as a force can be more powerful than violence. Sure, he may have to get into a brawl with the odd bruiser of the month, but fundamentally the first thing Superman will want to do is talk to the opponent and try to reason with him. That's why I'd like to think the villains of the DCU tend to respect Superman however grudgingly; the guy may come down hard on you at times, but he's a straight-shooter who'll at least hear you out.
The Spectre these days has become very, very 'Old Testament'. You break the law, you do anything to harm the innocent and you are going to hurt for it. The Spectre in his incarnation as the Wrath of God has become an aspect of primal justice, the current incarnation of the 'eye for an eye' philosophy also espoused in the actions of more violent protagonists (I hesitate to call them heroes) such as Wolverine or the Punisher. The Spectre's actions in dealing out divine(?) justice and retribution are brutal and oftentimes deadly.
The Spectre has considerable power with which to punish the guilty, but he has no real freedom either. He's the Wrath of God; essentially a form of sentient software designed for one purpose only: punish the guilty. Superman is a man gifted with powers and abilities far beyond those of mortal men, raised by loving parents and taught to believe in a world where truth, justice, and the ideals behind the American way of life are goals to hope for and rules to live by. Superman isn't 'born good' as was postulated in Denny O'Neils godawful Green Lantern novel. His DNA hasn't hard-wired him in the same way the Spectre's creation by God/The Source/The Voice has. He chooses the right thing. In a battle against Superman, the Spectre is essentially powerless. The only dent in Superman's moral armor Post-Crisis might be the execution of the Kryptonian Super-Criminals from the Pocket Universe, but given the jumbled state of current Superman continuity I doubt it even applies anymore.
In a contest of power between Superman and the Spectre, there's no question. Superman is powerful, yes, but he's mortal. He'd fare no better against the Spectre than he would in a prolonged bout with Captain Marvel. But the Spectre cannot defeat Superman because Superman is not the guilty. The Spectre couldn't attack Superman, and even if he did, he'd be destroying himself by defying the fundamental underlying rule of his own existence. It'd be like Kirk asking an evil computer to define love on Star Trek. It'd lead to one serious bang.
Mind you, the Spectre and Superman have fought (in the pages of the Spectre, in the Day of Judgement crossover, etc.) but I believe that to be due more to outside influence or the desires of the Spectre-force's host rather than the entity itself. Might poke a few holes in my above theory, but I think that it's kind of a grey area really.
I love this kinda stuff, don't you?
-Def.