Short version:

With modern ammo, the 9mm gives nothing up to the .40 or .45.
And extra bbs.

Secondly, the .40s don’t hold up as well as ths 9’s, the increased battering wears the gun out quicker. Its why the gen4 changes were made.
That’s also why so many PDs are replacing guns quicker.

There’s a couple of reasons the FBI and others have gone back to or never left the 9mm, its easier to shoot and shoot well.

And by getting a spare slide assembly, you can have your .40 later if you so desire.

I am considering getting a different teaching gun, my G30 is a little pricey to just blast ammo off with. I have owned countless 19s and 23s but able to fire all three side by side again at my last class. Bluntly, the 23 was the least fun to fire. I then remembered why I never kept them. Just enough harder to shoot that its work.

At this point, a 19 or grip shortened 17 are the front runners.
That’s if I stay with the platform, whichni probably will, if just for teaching.

And a decent barrel runs @$120.