A handy person from Ladera Ranch, CA says:
September 7, 2016
I have a 17 lb window, I can select a spring for 12 to 16 lbs, or the 16 to 23 lbs. The 12 to 16 lb spring makes the window easy to close, harder to open, the 16 to 23 lb spring makes the window easy to open, harder to close. For the long haul, which spring do you recommend? Will the lighter spring keep the window open? Or will it start to drop over time?
Tom from SWISCO responded:
September 8, 2016
Hmm... if your sash is exactly 17 pounds, I would stick with the 16 through 23 pound option. The lighter balance is likely to start droop eventually. Then again, having a balance that is too strong isn't any good, either, but with a sash of 17 pounds there really isn't any alternative. That is likely your best bet.