Power = cadence x force.
If you want to do your intervals at 300w or whatever, you can do that at any cadence you would want to pedal at. So a cadence sensor by itself isn't that helpful for training.
And because of the math, a PM in the crank or pedal has to measure cadence to know power. So there's no point in having the sensor if you want to go this route.
A power meter is much more expensive of course. But it's kind of the holy grail of things you could measure on a bike - because of its usefulness.
If you have a speed sensor - this comes down the line - you can do a trick riding laps and using free software to measure your cda. Test different jerseys, positions, etc.