PX4 or ArduPilot: The Real Fork Is the Licence
After cloning and building both, three things differ from the stereotype: ArduPilot's EKF3 header credits the derivation to PX4/ecl, so the hardest layer is shared; PX4's last year of commits comes from company domains (380 from Auterion alone) while ArduPilot's comes from personal addresses with one contributor at 37%; and what really decides the choice is not performance but BSD-3 versus GPLv3, and which layer you need to modify.