Pre-Crisis Cosmic Boy (but not early Golden/Silver age, as their powers were often low scale, relatively) was on par with Magneto, in most "realities"; Magneto, however, is much more clever/devious, and his powers have a wider range of application over other forces (though much less than with direct magnetism alone) -- this makes for Mag having the victory in most cases, though if it were something as simple as a tug-of-war over a ship or something, Mag would be quite surprised at how much resistance Cosmic Boy could create in a pinch.