Compared to new points and condensor, the Pertronix offers no more power nor better mileage. However, as soon as you start to drive the car with points and cond, they start to degrade. Your dwell changes, which changes your timing. This leads to reduced hp, and lower mileage.
The Pertronix offers rock steady ignition performance that does not degrade.
Although you might think a Pertronix is expensive, in reality it is cheap. Since the dwell and timing never changes, you don't need to own a dwell meter or a timing light. All you need is to borrow a timing light for the initial setup, then you are done for the life of the engine.