Parelli pads are completely flat and quite stiff, meaning that with a lot of horses they press down on the wither. The Best Friend has a little bit of a curve at the front, that doesn't completely solve the pressure with a high withered horse, but it helps. Best Friend pads are also more comfy to sit on and a fraction of the price.
(I had a PNH pad and sold it on EBay to fund a Best Friend).