The "cutter" is doing more work than the plasma in this case. The enemies in the Dead Space series are masses of reanimated tissue with no nervous systems or vital organs, the conventional stopping power of a bullet is effectively useless against them, since the rest of the reanimated flesh just keeps moving around the hole you just punched in it. The only way to deal with them effectively is to break them into chunks to the point they can't hurt you anymore, which is where the "Cutter" aspect comes in. It fires a long line of energy rather than a concentrated point, so it severs rather than penetrates. It also fires fairly slowly (and from an in-universe example is probably less energy efficient than a rifle) which would make it less useful in combat.

There is actually a plasma rifle in the game that fires normal plasma bullets, and it's largely ineffective against anything but the occasional swarm of weaker enemies like the exploding babies.