OK, I've just tested this by going for a run (the sacrifices I make ... 😝 ).
On the Watch:
- The Workout app records time, distance (but see note below*), pace, calories used. At the end of the workout, if you save it, it appears on the phone - see later
- The Heartbeat app records heart rate at intervals
- The Activity app records steps
* Note on distance: the Watch doesn't have GPS so, for accurate distance, it needs to be getting GPS data from the iPhone. According to Apple, the Watch learns, over a period, your stride length, so if you don't have the phone it can estimate the distance based on that. I don't know whether it's smart enough to account for changes in stride as you go up and down hill, but that's something to learn as we go
On the iPhone:
- The Health app records heart rate, calories, steps, distance (split into running + walking and cycling, steps, flight climbed (which also makes an effort at hills too). BUT it doesn't record these items against workouts - it records data points - readings at a point in time. So you can look at the Dashboard in Health to see the total for the day (or week or whatever) and you can drill into the individual measurements to see data points. But there isn't a place to record that this run was x miles at y pace in z time, ad my max heart rate was whatever.
- The Activity app mirrors the Watch Activity app AND any complete workouts that you've saved on the Watch with all the metrics you recorded
So between Health and Activity you get a good set of data, albeit (in my opinion) not yet in the ideal layout - but I'm sure that will change as Apple refines this. These are new apps, still on version1.
Final point - apps like Runkeeper and others have Watch versions, so you can use them to record more detail if you prefer.
Hope that helps