I'm trying to decide myself. I always thought it would be cool to have an SBR'd MP5 (PDW) style. It'd be short enough for corners and close quarters and have a stock and VFG for better control. Plus 9mm seems like a good compromise between power, recoil, and overpenetration. But between your standard choices (carbine, shotgun, pistol, pistolized carbine your choice of caliber) Really I'm jut trying to justify buying another toy I probably don't need.