Originally Posted by English
Much the same as above but get a SIG 357 instead of the .40SW. Still get the 9mm conversion barrel and two 9mm magazines. If you want to get the .40 barrel later that is good too and uses the 357SIG magazines.
It is a delicate subject around here but 357SIG is probably the best defensive round below the 10mm and is almost certainly better than the .45ACP both in effect per shot and rounds per magazine as well as pistol weight. The 10mm G20 is a very nice pistol and you can get .40 and 357SIG barrels for it but not 9mm. If you really want the .45, try to hire one first because the grip, like the 10mm is too big for many hands.
To echo English's comments along with mine I would add that if you are to use the weapon for defense, it is wise to purchase it chambered in the round you plan on using for defense and then you can "play" with the others at the range.
Both the .40 Glocks AND the .357 SIG Glocks can shoot each others caliber as well as the 9mm.