Second to Madsvg.
But to answer you question, I am partial to the 1911 and Glock platform. If you want full auto without the bulk of the mp7, see if you can get a hold of a TM G18c. It may be a bit of work, but it would be totally worth it. With an extended mag you could clear to your hearts content and still switch back to a compact field sidearm.
That being said, the mp7 is fantastic. I know a guy who has one (fairly big guy) and he wore it in a leg holster all day. If I did end up persuading myself to spend the money, I would do it. Now if you are under 6ft, maybe not. They are big weapons. Another alternative would be the smaller mp9.
But if you want a pistol sized platform, look into a TM Glock or Hi Capa. If I could choose another sidearm, it would be one of those. Hands down. KWA does make some quality pistols, they are just proprietary so upgrading is nill. To their credit, they don't break easily or often.
Now my final comment would be, you don't really need full auto even for clearing a building. That is why out of all the ones I mentioned, I would get the pistols. If you like full auto, you can get them with it, but really, a quality sidearm with fast response is all you need IMO. (If you look hard enough you can find Hi Capa's with full auto.)
Hope this helps.