Overall cost wise, the 9mm and it seems like most think the same way there.
But also, that means that finding 9mm is going to be harder too when panic buying occurs (as it is currently). So really the .40 is to me the smart move, you're more likely to find .40 ammo then you can get a 9mm conversion barrel for the .40 to shoot 9mm too, you can have the best of both worlds.
I like the .45, but it's not very cost effective unless you reload and frankly I wouldn't even consider a 357 Sig, for much the same reasons and then some.