In short the Polling Rate is essentially how fast your mouse updates your computer with it's position, the higher the number the faster it responds. Professional CSGO players and I personally use 500hz as 1000hz feels far fetched and too sensitive for human hands, 1000hz feels more shaky than 500hz. It also depends what resolution you are playing at.
In correlation to DPI this does not matter too much. People use 500hz because they feel as they are more precise with it. 500hz and 500DPI is a good balance, even 800DPI for native res players. Personally I'd recommend 500hz as it feels much more accurate when aiming. It is an indistinguishable difference but 500 is possible the way to go.