The s4 one is better but more expensive? Thd a8 one is cheaper and you keep a4 caliper and just replace carrier and rotor?

 

Correct.

 

Keep in mind the A8 upgrade fits under stock 16s. You'll need 17s or larger for the S4 brakes. Sheasta, depending on what car you have, there may be more parts you need for the S4 brakes. You need to make a sig with your car's specs.