20c 4s lipo or 30c 3s lipo??

[suicideneil]

More power doesnt always mean more speed. You need a certain voltage with a certain vehicle to be reliable and run cool, and attain a certain speed- a high C & mah battery will still be discharged rather quickly if you try to gear it too high (match the kv of the motor to the voltage you plan to run, then find suitable batteries for the amount of power you will need with that combo).

What vehicle?
What motor & kv?
What purpose/ intention?