The Penn ATP balls are an upgrade over the Penn Championship balls. In fact, Penn sells an 8-pack where they include 7 cans of Championship balls and 1 can of ATP balls, and they advertise on the package that you get 1 ATP upgrade. You can tell right away that they have a different colored felt, which is supposed to be better under night lights. I thought they were better than the Championship balls in terms of consistency and durability.
I do like the Pro Penn Marathon, which I think is their top-of-the-line ball. If you can past the fact that the regular felt fuzzes up early in the first set and wears down after that, it's a great all-around ball, but the Wilson US Opens last longer.