1. SS springs are stiffer.
2. SS springs make the car sit higher.
3. I will let the others take car of that, they will give a more informative answer.
Which and why:
Which: SS springs if you go to the track and drag race, competitively, all the time.
Why: Because SS springs are Drag racing springs, that's what they were designed for.
Which: XHD for everyday and/or a street/part-time strip, car.
XHD springs are advertised as a stock replacement for the original heavy duty springs that came on performance models (or cars ordered with heavy duty suspension). XHD springs can be ordered for extra lift. +1", +2" for example.
Why: They provide better handling and a better ride while still being a solid spring for Drag racing in a hot street car.
