Max Heart Rate during Workout Apple Watch
You don't need any third party apps to have your heart rate sampled all the time and displayed on the watch. You can customize each workout type to include or exclude a dozen metrics for the Outdoor Run type. Indoor run is the same and I don't use it, but the treadmill integration is amazing from what I hear when you sync your watch to equipment that will visually show you what telemetry the watch is broadcasting (with your opt in permission, of course).

Here is what the phone interface looks like for you to alter what is shown during the workout image above.

So if you didn’t care so much about pace, you could edit out both pace items and see this instead during your run:

Garmin makes some great product, so I would choose based on where you want your data in the end - which app ecosystem is better for you. Heart Rate zones are going to be easy to set / manage / report on both platforms as it's so obvious and easy to measure.