If you like reading crazily in-depth reviews, please see DC Rainmaker:
https://www.dcrainmaker.com/2017/07/best-bike-gps-2017-garmin-wahoo.htmlIf not, please ignore!
Note on the Garmin GPS. I have a Moto G 3rd Gen and it picks up the satellite faster than my Garmin 500. Even a dedicated bike GPS will occasionally behave very strangely - particularly on mountain climbs shaded by trees it seems. Also, every now and then the Garmin takes about 15 minutes to find the satellite - whereas the phone never has an issue.
In short, if you are keen to improve your cycling and you love the data, then a dedicated GPS with cadence and heart rate is definitely fun to have, and will help you increase your strength and endurance, but it's by no means essential.
- Knowing your cadence at any given time and trying to hit a designated sweet spot can help your training - but then again, everyone's different and some people spin at a high cadence whereas others are better low.
My cadence magnets fell off and I'm waiting until the the revised sensor goes on sale before I buy another one. I'm surviving fine without it.
- Heart rate is great for training in different heart rate zones - but you need to understand the zones and develop a training routine to get the most out of it. I enjoyed wearing the heart rate monitor when I went to bed one night. Seeing the spikes in heart rate during REM sleep was pretty interesting.
Ultimately you can get fitter and stronger by using a GPS with cadence/heart rate tools to optimise your efforts. Or alternatively you can develop a route you enjoy and just keep on trying to beat your times. Neither way is the right way - just depends on what you enjoy more.
I don't race but I regularly ride about 200-350kms a week, commuting, trail riding and climbing hills.