This has been discussed to death and really the only consensus is try all the coombinations and go with what you like the best.
I have SS+ AND 17" with Run flats. I like the firm ride, some people say its to firm, I know people who say I need to get it stiffer!!

(They are a Girl Racer who drives a moded Clio RSi though)
We can argue about which is the best for performace but the fact is its how YOU like it, nit us. Facts and figures, states that SS+ is 10% stiffer than SS I beleive.