my $.02
steel mags: heavier, prone to rust, prone to denting, prone to corrosion, need to be oiled/maintained in storage, need to be oiled/lubricated on inside, older style - many in the us, most are older in age than polymer mages, not as durable, most wobble somewhat in mag-well, make more noise, inconsistant finishes/product, harder to clean, sharper edges...
polymer mags: lighter, not prone to rust, not prone to denting, not as prone to corrosion, don't need to be maintained/ oilked in storage, don't need to be oiled on inside (self-lubricating), newer style, - not as many in the us, most are newer than steel mags, very durable (Bulgy waffles), pleasing to the eye, minimal/no wobble in mag-well, less noise, consistant finish/product, easier to clean, not so sharp edges...