Just gonna throw this out there from a reloaders perspective.
I have a G31 in 357 SIG with an aftermarket barrel in 40 S&W and a 9mm conversion barrel.
I reload for each caliber, for 9mm I load light loads with cast bullets for range practice and all around fun shooting. In 40 S&W, I ramp it up but still use cast bullets, this way I get a feel for SD ammo recoil. In 357 SIG, full power reloads with a jacketed bullet duplicating SD ammo, you really donít want to run a cast bullet in a stock Glock barrel at 357 SIG velocities. This all works for me because I reload and have no real worries about what or what isnít on the shelf of ammo retailers and can practice with whatever ammo I like.
I decided on 357 SIG as my SD round and would suggest that you decide on which caliber you plan on carrying with and keep your Glock stock in that caliber, then get whatever caliber barrel you like to save on cost for practice ammo.
Just my $0.02 worth