The G400's optical sensor is able to track consistently at a high rate of speed on most surfaces. Highest on cloth. The G500s laser sensor is able to achieve a higher maximum count and max IPS, but not consistently on soft fabric or dirty surfaces.
Why is this?
-Laser imaging is extremely sensitive.
In terms of consistency, a clean hard surface will keep it's high performance stable. Any dust, hair or dirt in between the sensor lens potentially gimps the sensors incredible tracking rates.
-As much as I don't want to claim that a sensor is "accurate", the G400 is technically superior when it comes to high speed point to point movement due to the 9500's small accel flaw.
That at least is the technical side of the matter.
-If you want my true opinions, I would indeed take the revised 518, but that's not to say that thousands of people aren't happy with the G500.
-If you use your wrist to pivot a mouse, I doubt everything from the acceleration to max tracking rates will even be a concern to you.
-I'm not trying to scare people away, but I will do my best to inform that there are potential flaws whether they effect you or not.
-If you like the G500 then go for it. there's nothing worse than trying to imagine what could have been