I'll have a go at it.
Both suspensions are basically identical except for:
Sway bar mount - SC factory bar is under the suspension. Early bar is through the body.
Control arms - Due to the sway bar mount, SC arms have a different configuration to mount the sway bar end.
Crossmember - SC is aluminum. 73.5 is steel
Brakes - Same unless the 73.5 has M brakes which have a 3.0" caliper mounting pattern (and solid rotors). If the 73.5 has A or S calipers, the spacing/brakes/rotor are the same.
Sheet metal cover under steering rack area - Different
Struts - Early may be Boge, Koni or Bilstein. SCs are Boge or Bilstein.
Age - SC is newer, maybe less wear to wheel bearings, ball joints
Someone should verify my attempt to clarify the above.
Sherwood Lee
http://members.rennlist.org/911pcars
PS: If the struts are Bilstein, replaceable w/Bilsteins. If the struts are Boge, can replace w/anything.