No experience with the PX4, but I love my M&P 9. I'm a lefty too, and the completely ambidextrous controls make it a lot easier to quickly operate it. The only thing that seems to be common with them is that the trigger feels "gritty" for the first few hundred presses. But after that, it's smooths out nicely.
But as far as overall quality/reliability goes, I don't think you can go wrong with either choice. Have fun making your decision!