Depends...the effort you are putting in to pedaling at a low cadence...are you just pedaling at a low cadence because it is easier or are you pushing the power to work the muscles?
Generally spinning at a higher cadence works the cardiovascular system harder thus a higher heart rate versus watts.
If your intent is to increase power then pedaling at a lower cadence but with a high exertion of power will result in a higher power output and increased performance over time and effort. But if you are just pedaling along at a lower cadence because it is easier to do so the performance will be decreased because the effort under load isn't there.
Same with a higher cadence. It is harder to ride at a high...over 100rpm...cadence over a period of time because of the exertion on the cardiovascular system. But with training your cardio system becomes adapted and better able to achieve a higher level of power and cadence.
It really depends on what you are trying to achieve and the effort put forth to achieve it.