It makes no sense for magical beings with magic invulnerability to be just as weak against magical attacks as a being who's invulnerability isn't magical in nature. The way I see Superman and Shazam's power levels are that Superman has better strength and suplimentary powers (hearing heat vision etc), but Shazam is more durable against certain types of attacks and his one distance attack is magical and thus can harm Superman. speed wise they seem pretty even to me.