Steel tends to be heavier, and depending on % carbon in it determines the grainstructure of the steel itself, thus determining the strength of the steel. High carbon steels are incredibly tough yet incredibly brittle as they have a austinetic/martensitic grain structure(depending on treatment), you dont tend to get that with aluminium as its a more malleable/ductile material.. Things like welded joints on the frame will have a degree of effect too, with both steel and aluminium as if its not treated the heat affected zone will have completely different composition to the main material itself so its swings and round abouts really.
