polling rate is basically the response time of a mouse. higher the polling rate the more pings it sends to the computer per second, increasing it's responsiveness.
G500 is specifically better cause it's more ergonomic, has a superior sensor, and more buttons. Really that's it but those three things are the core components of a mouse.
For me personally, it's all about the responsiveness and accuracy, with cheap mice like a microsoft one, you point your mouse somewhere fast, but it doesn't always go where you want it to go. Where as with the G500 and G500S as examples of good gaming mice, they have features that allow you to more your mouse where you want it to go with pinpoint accuracy.