The Division for both IDPA and USPSA is Carry Optics, not Carry Comp. And IDPA is not training, it's a gun game just like USPSA. You should at least read the rules of the games. Like Zero said, don't be that guy. Get a straight drop belt loop OWB Kydex from a competition holster company, BladeTech, CompTac, Redhill. And a competition belt, like Wilderness Tactical, not a Wally World belt. And at least 3 model specific to you gun mag pouches, not fit all.
GSSF is not action pistol. The only purpose of their holster is to transport the gun from stage to stage. Absolutely do not use a Glock sport holster. It is nearly impossible to draw from. And no retention holsters for action pistol, only for PPC (Police Pistol Competition.)
Shooting is 99% the shooter, 1% gun and gear. Most shots will be 3-12 yards so your G19 will be fine for both games. Since dots are target focused, a G34 has no advantage over a G19 in CO Division.