The mainspring determines how hard the hammer falls. The rebound spring determines how snappy the trigger return is. To get the lightest action, you want the lightest mainspring that reliably sets off the primers, and the lightest rebound spring that reliably returns the trigger with the mainspring you're using.
Determine the mainspring first, then find the rebound spring you like to go with it. I's a balancing act. You can't have the lightest trigger return spring with the heaviest mainspring. If for self defense, use the heaviest hammer spring no matter what.
