I got my G20 off another CGN'ner and it came with a .40 and .357SIG barrels (both 6" lonewolf barrels with 2 ports on the top of the barrel)
I prefer to shoot with the stock 10mm barrel, but have used the .40 barrel with great sucess. It's as accurate as the 10mm barrel. Have not had a chance to try the .357SIG barrel, since I have no access to the ammo here in Winnipeg.
I too like the idea of using the .40 barrel more since the ammo is much cheaper.
Later..........