If you already have a Ruger 9mm pistol look at the Ruger PC9 carbines as they take the same mags. The downside to the Ruger is a heavy trigger pull that can't be easily lightened.
After shooting with some guys at our local match that doesn't allow rifle calibers I bought a Vector Uzi.
Advantages are: designed for 9mmP from the ground up, compact even if you don't SBR it, easy to SBR (I'm waiting on my Form 1), looks evil, spare parts and mags are available and cheap, and it has a retro cool thing going. It's also surprisingly accurate, mine gets 4MOA groups and the range adjustable sights make hitting targets at 200 yards doable. Did I mention that the Uzi is built like a brick outdoor lavatory?
The down side is that there just aren't any real good ways to mount optics on an Uzi and it is heavy. OTOH, a 8lb 9mmP weapon has almost no recoil which makes it a great training tool. BSW