Consistency and flow rate.
Usually presets are a bottleneck and you will experience drop off and the like while shooting rapidly. With a good adjustable (dynaflow, conquest, hyperflow, maxflow, armageddon, a.i.r) you won't experience it because they are higher flowing regs that regulate quite consistently to create less work for the regulator on the gun.
Consistency is a by product of a good/consistent/highflowing reg to provide the reg on the gun with sufficent air so taht it can do it's job. If you have a reg that doesn't flow as good, you will experience drop off which means less gas to propel the ball which = lowered velocity.
My recommendations for tanks are as follows (no specific order):
Dynaflow, Conquest, Armageddon, Hyperflow, A.I.R.