I believe the only real difference between the two is the sensor. Functionally they operate the same, but the HERO16K sensor requires about 10x less power. In a wired mouse this won't make any difference whatsoever. But in a wireless mouse, it will make a huge difference in battery life. Unfortunately, at the time of this writing, the only wireless mouse with the HERO16K is the Logitech Pro Wireless. The G603 has the HERO12K sensor.
the G502 HERO is the updated version of G502 Proteus spectrum , Logitech promises 16,000 counts per inch (CPI) and 400 inches per second (IPS), but as usual, those numbers don’t tell much of the story when it comes to a gaming mouse.