Well you have a G21 so the G30 makes sense as they're both 45ACP. Good hard hitting caliber but not excessive recoil. And you can share the 13 round mags. You really can't go wrong.
Still, the G23 is my favorite Glock and also is hard hitting with manageable recoil. I really don't see the 40S&W as slower back on target than 45ACP at least in a lightweight like the G30. I have a 1911 but wanted more capacity though wasn't ready to drop to 9mm so chose the G22, G23 and G27. It's worked out well for me plus I can practice with 9mm in the last two using a G23 40-9mm conversion barrel.
Anyway, both 45 and 40 are a great choice.
The biggest issue with assembling an AR isn't so much getting the parts together right - it's getting the right parts together.
US Army 1966-69, VFW Life Member, Retired Geek