Buy the 40 caliber.. For a Glock 400 rounds means it’s hardly broke in. The only thing I’d recommend is to put the pinky extender on the magazine; I have it on my G26 and love it. I’d also recommend buying a quality leather holster.
As to caliber: 40 caliber vs 9mm difference are minimal, in both recoil, and ballistics. Of the non magnums, it’s more important which SD ammo you use that caliber.