An ideal computer, even a simple one, is weatherproof, easy to use whilst in motion, and with a display that is easy to read in bright daylight, and always on. An ideal bike computer will consume battery power slowly; it will last longer than your longest ride, possibly all season (except GPSs, which still need to outlast your longest ride). And an ideal bike computer will be relatively accurate even at low speeds.

A cell phone's screen is usually not always on. If it is, its battery won't outlast your longer rides. A cell phone isn't simple to use while in motion. And with the GPS in higher accuracy modes the battery will draw down quickly. A cell phone's screen may be hard to see in broad daylight. It is possible that you can find a cadence sensor hat will pair up over ANT+ or Bluetooth with your phone, and if cadence is all you want it will work, if you don't care to keep the screen on.

If it were me I would look for an inexpensive dedicated computer with cadence sensor. The cheapest I find online is about $45, and it simultaneously meets all of the criteria I consider ideal for a simple computer. Compare this to buying a $35 sensor that pairs with your phone. And then buying a $20 cell phone mount for your bike, and a $5.00 app in the app store. $45 for a computer that is ideal, or $60 for a cell phone solution that is not ideal. Penny wise and pound foolish.

If you should decide you are willing to spend more, a Garmin Edge 25 is around $100, plus $35 for a cadence sensor. For $135 you get the breadcrumb trail, uploadable (but not self correcting) routes, data sharing with Strava and Garmin Connect, and other apps. I think that's a great value. There's a lot of utility in being able to look over a rider's characteristics after the ride. But that's just me. For your purposes possibly just a $45 computer with cadence sounds appropriate.