4-bar vs. leaf springs

[Hertzdaddy]

I am curious about the performance differences between a 4-bar rear suspension, like you get with most of the air ride setups, vs leaf springs. I've considered a full-on 4-link system for my 73 Jimmy but I'm not sure I want to take on a project like that. A bolt-on (more or less) 4-bar/panhard system looks appealing if performance gains are to be had. Lets assume coilover for the 4-bar suspension in this case. Frame is already notched.

Is there a great improvement in ride quality?

What about traction in higher horsepower applications? Would a Leaf/Caltrac system perform better?

What about strength?

Other considerations?