As a general guide, use the heavy spring for anything over 1250fps, and the light for anything under. This came from a guy that goes by the screen name of Mississippi Dave over on Sig Forum, and has proven to be pretty reliable. He also has an awesome post regarding the Mosquitos and some of their quirks (especially the early models): Sig Mosquito Fixes, Condensed General Version - Topic Powered by Social Strata
Using his recommendations, mine functions nearly flawlessly with the Remington Golden, CCI Mini-Mags, and Federal bulk packs. Since mine is pretty much broken in, I can now leave the heavy spring in full time with no problems.