If at all possible go to a gun shop that carries a good variety of Glocks and handle the models in the various calibers you're interested in.
The service caliber size pistols in 9mm and .357sig/.40S&W are all the same size so even if the shop doesn't have a G-23/G32 handling a G-19 will give you the same size pistol in those calibers.
I've never seen a G-21 but have held a G-20 once and it's far too large for my hands. Also never held a G-30 but the G-36 felt okay to me if I ever wanted a small .45ACP.
The only Glock I own is a G-32. I once considered buying both the 9mm and .40S&W conversions but like .357sig so much I gave up that idea. It's extremely accurate and the recoil is just a tad stronger than +P+ ammo through my 9mm subcompact. Don't believe the myth about .357sig having too much muzzle flip. The Glock I own has no more muzzle flip than the 9mm subcompact; it just has more snap (the only way I can describe it).
Ammo costs are lowest for 9mm and it's quite capable for self-defense, but if I had to choose one caliber it would always be .357sig.