Both have the GM 4718M Performance Specification, (Corvette/Cadillac approved), While Valvoline only recently got this Spec in 2009. PP I'm pretty sure has the Honda Spec HTO-O6 while Valvoline does not as of yet. Not many others are HTO-O6 approved, I know Mobil 1 is... not sure of others.
So, between the two, if same price, I would lean towards the PP just for that fact.
Actually, I've been leaning heavily on the Q Horsepower Synthetic at Walmart for $18/5quart jug! Stocked up on 4... and counting!