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.
If the magic weakness was defined that way (which is not an opinion all writers share), then it would mostly likely BE written that he avoids getting hit using the various means at his disposal.
Can't have a dead Superman after all!