WHATEVER YOU DO, DO NOT GET THE ELECTRIC MP7! NEVER! Even the TM one is crap. The mini-gearbox is hard to work on, and repacement parts are harder and harder to come by.
However, I have heard good stuff about the GBB model by KWA, but Ive never held or shot it in person.
In terms of a good, small, sidearm, I would go with the Echo1 or Galaxy mp5k (Its an AEG). It is an incredibly small gun, and mp5 mags are cheap and pretty plentiful. The reason I would not go with gas is because you have to continually supply gas, and Ive heard the Well G55 isnt great in terms of reliablity. (Not speaking from expirence with the G55, but I have used a Galaxy mp5k with good results)