Opinion: Been toying with the idea of x2 48s or pick up an extra 21 even though I do not shoot 45 at all. Long debated 48s cuz of 9mm availability and indeed if anything ever breaks over the next 40 years lol. I get they are not fully compatible models of course. Thoughts?
I really hate getting into detail on these things on the internet. It's way too easy to appear as a wingnut prepper
It really depends what else you have, what you expect from the future and what you are buying for. In my case, I have a G17 & 34 and a spare complete frame, full parts kit, and spare 34 barrel. I also have plenty of 10/19 & 10/33 mags, and a Ruger PCC. If the sh1t truly hits the fan, these would go along way. And a G19 as well, all Gen4.
I also have a 48MOS, but only 1. It is not my apocalypse gun, just a fun shooter, and CCW if/when I get my NH permit again and visit the USA. If it breaks, it is somewhat expendable if I can't get parts.
Competition I prefer 9mm 1911 to Glocks, so I have a pair of STI.
I also have a G22, G35MOS, G21 & 45ACP 1911 and extra mags for all. Although last week I considered all of these almost priceless, this week I am giving a lot of thought to selling all 4 of them. None of them get shot often, and with the freeze looming it's a lot of money tied up with no plan to ever move them to primary shooters. If the ban doesn't happen, I can always re-invest in them again.
So for your question, I think it depends what else you have, what you shoot now, and what you expect to maybe one day need. Without knowing your whole collection, I'd rather have 2x48 than 1x21, but if you already have a 21, spare mags, and ammo that's a consideration. If the 48s will be your first and only 9mms, I'd definitely say buy two (two is one, one is none

) but as my only 9mm Glocks, I'd rather have two 17s, 34s, or even 19s, and even more so if you have a Glock mag carbine.