Tass,
I have the Trade Secret, which to my knowledge is the same product as the PPG. We tried some on my sons white Sport Trac, which already has P21S on it, and really couldn't tell much of a difference. Probably a slight edge to the P21S...... but you had to look pretty hard. Now, on my Dark Shadow Gray SuperCrew, I actually like the Trade Secrect a little better than the P21S. On my wife's black CRV, Souveran is the champ!
Like Jeff said, UPP, or any other "optically clear" sealant is tough to beat on white!