It depends what your intentions are. If you want to hold up a gas station, pistol is just fine, range will be close, conceal-ability is important.
On the other hand, if I was going to show up at Times Square on new years eve and do some damage, I would pick an ar 15 with a 90 round drum magazine. If you want to go up in a building (stand off a way from your intended target) then you don't want a pistol, you want a rife and a few 90 round drum mags. Like the one just below:
Mag AR-15 Drum 90 Round MWG Clear Plastic Includes Stripper Clips and Loader
The Beltway sniper in 2002 used an ar 15 fired by an accomplice from a trunk of a car, and they were darned hard to spot, locate and apprehend. If they had cooled it instead of killing so regularly, they could have kept it up for quite a while. With a pistol, you normally don't get to stand off that far, due to the pistol being less accurate at longer ranges than the ar 15.
Beltway sniper attacks - Wikipedia, the free encyclopedia
I consider the ar 15 with 90 round drum to be far more dangerous if your objective is to go out and kill a lot of people, especially if you don't want to be detected on the first killing, kill a little from a standoff, and leave that hidden location and come back to kill another day. That is more of a war model, and not a practical theft model, which is the only reason that pistols kill more folks than assault rifles in peacetime USA.