I have a Sub 2000 that takes my Glock 19 magazines - thinking along the same lines as you. I have no complaints, it works as well as any 9mm carbine or subgun. The ergonomics aren't as good as an MP5 but the reliability and accuracy is good to go.
My initial Sub 2000 was one that took Beretta mags - I don't have a Beretta but the deal (at Firearms Guild in Rahway) was far too good to pass up - way below dealer cost for a very lightly used gun. It fell off a desk (in the unfolded position) and the front sight tower had a big argument with an AR front sight tower, which it lost. EMAILED kel-tec, received a reply "send it to us, we'll fix it". Sent it there, got it back in about 2 weeks with not only a brand new front sight assembly but a whole new barrel. Only got rid of the Beretta one when I got the Glock one.
Now that being said, I do not like the cut-down grip of the G19 model as much as the larger grip of the Beretta and G17 S2K models. It works best (most comfortable) with the older style G19 hicaps that fill in that little gap in the front. But it's a small price to pay for the ability to interchange mags with the G19.
If you owned a Beretta, I'd say consider the Storm, it's a nice gun. However I don't see a reason to spend $600 on a 9mm carbine for "SHTF" purposes (in other words, I hope you already have an AR/AK/HK/FAL type rifle before buying a 9mm carbine :). The main reason to me to have a small 9mm carbine is for situations where I'd rather not have a full size rifle or M4 type carbine with me. And the concealability/portability of the Sub2K cannot be beaten, unless you've got an MP5K or a Micro Galil :)
I'll try and post a picture here later today.