p = pivot heel, where the heel twists when you eject, very safe
px = non-pivot heel, much more like a normal binding where the heel is fixed
i've only ever had the p series but I'm putting px 12 jibs on my watchlifes next season to see how they are, look knows what they are doing and I doubt they would make a crappier product, people are just pissed because they loved the p series.
and the reason that there aren't many small binding companies is that a binding is very, very hard to engineer, especially one as safe and good as those that are made by the big companies. thats why 4frnt, amplid, and a few others have all just rereleased the vist bindings.