Galloway's website states that the factory spring rate for both the 40 and 9 is 16 lbs. they offer the 14 , 18,20, and 22 for lighter and heavier charges in hand loads. If you are buying comercial ammo and not reloading I recommend the 16 lb. I ordered the stainless 16 lb for my 9c and am very happy with it. The stock guide rod would bind up and was hard to reinstall. The slide was stiff to rack when I first got it and is nice and smooth now. I don't know how much of that is due to the guide rod or just due to firing several hundred rounds. Enjoy shooting your Ruger, I love mine.
And welcome to the forum from North Carolina