Is there any performance difference between the PX4 and the Ardupilot, for a hex or quad configuration?
I recommend Ardupilot for most users, as it has more features (including safety) that most people would use. In terms of performance, there probably isn’t much noticeable difference between them for a well-tuned medium-sized copter, but Ardupilot might have a slight advantage thanks to the dynamic notch filter. But it’s definitely easier to get a good tune with Ardupilot.
The advantages that PX4 has is mostly for research/simulation, specialized applications, or if you want to sell software based on it (Ardupilot is copy-left under GNU public license).