i don't think it matters much

up to you to test it out and report back!

It has no effect on the max speed the mouse can track on, but it has effect on smoothing the movements, which is why the polling rate change was intended for, in first place.

Like douille wrote above me, the mousescore test found out that the older MX 500 got betther results if you changed the polling rate when it comes to it's maxspeed. Pretty interesting because it should have worked just the same at 125hz but didnt. You can almost call it a bug. Altho the main reason why you change polling rate to 500hz instead of 125hz is because you get a smoother feeling when moving your mouse, because windows checks and updates the mouse position more times then at default.
And that effect is what you will get from MX518 and all other mices.

it's because mx518 uses a 16-bit channel instead of the 12-bit that mx500 uses. I think this was mentioned in mousescore

it means its good out of the box ;P

Hz*