Another vote for 23# mainspring. 16# is sort of the standard for 5 inch model recoil spring, but an 18# will give you a little more longevity, especially if you use hotter +P loads like the Federal HST. Just make sure your gun is reliable with the heavier spring.
"The 1911 was the design, given by God to us through John M. Browning, that represents the epitome of what a killing tool needs to be. It was true in 1911 and it's true now." - Col. Robert Coates commanding, U.S. Marine Corp Special Operations Command Detachment 1 (DET 1)