Although I have a bunch of 45 ACP guns and ammo I went with the 9mm.
I did because I have a Beretta Cougar 8000 which you could get the Storm carbine with the same magazine. I don’t believe you can any more.
I can’t really come up with another reason. I’m glad I did. Mine really likes lead bullets and the cost for me shoot 9mm is very low.
If you reload for the one you get keep the loads in the upper pressure level. The gun has a lot of mass for the bolt and it can have some ejection issues if you stay in the lower load levels.
My guess is that it’s had well over 2000 rounds and for plinking it is by far my favorite pistol caliber carbine. My MP5 is boring in comparison.
I would suggest you get the one you have the most ammo for. I will warn you, you will eat up whatever you have in the ammo bucket.
"The difference between stupidity and genius is that genius has its limits" Albert Einstine