Get some of each! They are not too expensive. That way you can swap for the ammo impulse your intended on using. Higher impulse ammo also tosses the brass in a low earth orbit, the extra power springs tend to help them stay a little closer in. Some have had issues with certain types of aftermarket springs in the g-20's, so check and search to see who is getting best results with what type. Maybe some one could shed more on the G-20 RSA for you.
I use the WolfF Gun Springs two piece recoil rod and dual 21 lb spring set on my Glock 29. I also use the 22lb on my S&W 5" guns.
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