The factory weight for a 9mm is 17 lbs., and for a .40, it's 20 lbs.
Unless you're having cycling problems with lighter loads - such as the gun not feeding or extracting properly with lighter rounds -- there's really no reason to go to a lighter recoil spring. A lighter spring will make it easier to rack the slide, but you probably won't notice a lot of difference, otherwise.
Wolff has a lot of options, including some variable-rate springs.
www.gunsprings.com