Superman would probably lose, due to his vulnerability to magic. According to Byrne, Thor hitting Superman with Mjolnir would be like Lou Ferrigno hitting Arnold Schwarzenegger with a sledgehammer.
John Byrne: "Superman has no special vulnerability to magic. He just has no defense against it, like thee and me. As to the question of Thor's hammer, it would be like Lou Ferrigno hitting Ah-nold with a sledge hammer."
Theoretically, Superman could avoid all of Thor's strikes with his super-speed, but it probably wouldn't be written that way. If Superman didn't have that weakness, he'd probably beat Eric.
good point, and John B was also the one that stated his Glads mop the floor with that Supes, bujt was very surprised when he came back and did a few issues withj Gail Simone, as DC to him had him getting back to PC like levels again!