For self defense,Magneto's are the best, for someone who cares about the whole world , Susan's force fields are better, magneto's force fields are useful for protecting himself, but they're no impenetabre, Shadowcat can go in his force field, and she can't go through susan's, for they have no molecular structure, susan's force fields can protect cities, not only herself, like magneto's, her force fields saved a city from a tsunami, and from a nuclear and a gamma blast, she already held the gladiator, the hulk and the thing at the same time,also pushed Hulk alway with one simple blast, Susan's force field's has already gone into the watcher force fields, and her force fields are in celestial level, she already blasted through a celestial armor, blasted exitar, and YES! she has already blasted through galactus!
and magneto's force fields in a wider range gets a lot weaker, like in the first issue of the x-men, where even cyclops was able to blast through it, Susan's force fields are way better than magneto's.