I second pistol with tailhook. This way you get the folding stock and none of the SBR or 922r requirements. It's the best of both worlds. If the Scorpion was available in 10mm, I'd say the carbine was really worth thinking of, but for 9mm it's does not offer much benefit for the increased length.
Also, going this route allows you to SBR easily if you ever really just desire to do so.