Laser weapons tend to fire very quickly, very accurately, and do low damage per shot. DT is the bane of laser weapons. But against lightly armored/unarmed targets, they can cut down pretty large groups of enemies at range due to the high accuracy/DPS combo.
Plasma weapons fire much more slowly, have worse (but usually not terrible) accuracy, and do very high damage per shot. Within any given "tier" of weapons, plasma weapons tend to be the most capable of burning through DT.
EDIT: Plasma projectiles have been accelerated greatly in F:NV, moving about twice as fast.
