I'm a long time happy Abvio (Cyclemeter, Runmeter, Walkmeter) user who likes it as much for watch as I did for phone. It's the same app with different labels and is great for all the activities, and your choosing if you want to share with something like Strava or not. It will use your watch's sensors if you allow the access to health data.
My wife uses S2 alone much more than I do.
I don't think there's any right or wrong. It appears battery life will be an issue depending on the duration of your activities when you have the watch do it all. I've done 3+ hour MTB ride and just shy of 8 hours of mountain skiing and had battery left with watch and phone using the Abvio app. Two hours+ of hike or bike with my wife - her watch only, me with phone - and I see her watch battery use is higher.
I have Runtastic and others but haven't taken the time to try anything else with the watch because I like the Abvio app so much for all the different things I do.
On results? Yesterday we did the same 5 mi hike but my wife was watch only. My watch/phone combo said .06 mi more. I'm not sure one jaunt off the path with the dog was .06 mi. The Abvio app got more accurate with calorie burn after having the HRM. Walking around the pond near our house gets similar stats anyway we try it but always more distance after we got Apple Watches.