I think both are excellent series (though I always modify PLB especially by putting in longer breaks between sets, and the GS by putting in longer breaks between body parts).
I've always found PLB to have "just the right" lower body exercises, without anything superfluous, even without doing the stability ball work at the end.
PLB and GSL are currently duking it out for the place of "favorite lower body workout' in my collection.
And as Laurie says, PS and GS are more similar, both as three-day splits, and in their feel.