The stock componants are made from proprietory alloys, some contain more base metals than just iron which most aftermarket are made from. You can treat the aftermarket stuff with clear heat paint in an effort to make it not as succeptible to rust. Personally it doesnt matter as far as performance or longevity, more of a cosmetic feature. DOnt sweat it but if you start pulling to one side more than the other you might want to change both sides to the same brand or manufacturer.
95 E34 530I V2.37
===========
Those who make peaceful revolution impossible will make violent revolution inevitable.
John F. Kennedy