I've got a G20 too and a few thousand rounds through it.
For ordinary weak target ammo the 17 pound spring they come with will work fine. When you step up to somewhat hotter stuff go with a 20, and for really hot stuff like Buffalo Bore or Underwood, use a 22 pounder. 10MM ammo comes in such a wide variety of power levels that there's really no one spring weight that could be considered the one choice for all occasions.
Whether you use a metal guide rod or the stock plastic type never changed the way mine performs as far as I can tell. A non-captured metal rod sure makes it easy to change springs quickly if you're only going to have the one rod, but the easiest way to change springs (by far) is just to buy a selection of stock plastic captured rods and just leave the different springs permanently installed on the rods. The extra weight of a metal rod on a G20 really doesn't change the muzzle flip enough to be noticeable.
Really hot 10MM ammo DOES need the heavier springs. That's for sure. Weak springs will allow the breech to open to soon and your chrono will show less consistent velocities - meaning many of your shots will have less velocity than they would with the 22 pound spring.
Last edited by Brian Lee; 11-04-2012 at 15:07..