Pivot by leaps and bounds my friend...the PX is crap.
Yes it has the same elastic travel, blah, blah blah...the difference is in the way the elastic travel is achieved...the PX is all in the toe and 'feels' a lot different. The PX is also heavier and more prone to breaking. And the mount pattern is way bigger, which messes with the skis ability to flex as it was intended to. They switched because it was cheaper to make the PX, same reason they switched tha race-stock bindings over...because it's much cheaper to streamline the models and use the same machinery to make more than one binding.
I'll hoard P-series and FKS all the way, but when they are gone, I'll most likely look to Salomon for the STH, or some other binding. The PX just isnt worth it, the only reason I started riding Look was because of the safety of the pivot heel.