The main differences between a rifle and a pistol is that:
1. Rifles are more expensive.
2. They are nearly impossible to carry covertly.
3. They are more accurate at longer ranges (say over 30 yards).
4. They generally produce high muzzle velocities with the same type of cartridge.
If your shooting "technique" is "spray and pray", the accuracy piece is largely irrelevant of course.
If I were looking to kill and wound a lot of people at close range, I'd probably start with a shotgun (presuming I can get a shotgun into the venue) to create panic and then go to pistols with 15+ round mags.