So most people would feel power is more important than measuring cadence, so I would use it as a speed sensor. It depends on the kind of training you are doing whether HR is better or power is better. For lower intensity, lots of people like to target Zone 2 heart rate. For sustained long Zone 3 intervals HR can be good. For short intervals power is much better.
Here is what google shows me for your power curve
Looks like 15mph translates to 175 watts or so and 30kpg translates to about 460 watts. I would find one of ours that roughly matches this. This is not high precesion but it does not need to be. You just want approximates because if you take a ramp test it should make your FTP using the same approximation as your workouts so it ends up fairly close.