They are both pretty good. But they both have different uses. About the only time a plasma pistol is good is overcharging it to take down a shield. The M6C however is a good gun to use to get a kill after the shield is down. Might I suggest dual wielding with both the Plasma Pistol and the M6C for a quick kill.
The plasma pistol is what is needed to take down shields then switch to your battle rifle to take them out.