I use and love both, but on my red vehicles, the Souveran definately has some more "pop" to it.
I waxed once each during the past 2 weekends, the first weekend I used P21S(S100) over the Gloss Enhancing Paintwork Cleanser and the truck looked spectacular, but I've done some better work before on my Thunderbird with Souveran.
Sure enough, the following week I used Souveran over the top of P21S and OMG!!!! The truck actually "popped" before my eyes.
Part of this enhanced shine may be attributed to the second coat of wax applied, however, I believe Souveran had a large part to do with it and I think a topper of Souveran will make any red car look better.
Right on the jar, Souveran claims it's best for red and black cars.
I believe that now.
P21S cannot be beaten for ease of use.