I'm currently running with both a Garmin 620 (G620) and an Apple Nike Watch (ANW). Here's a few things I've noticed, with a focus on what might not be apparent from the specs.
- ANW consistently reports slightly shorter distances than the G620, circa maybe 0.05 miles per mile. This seems to hold true whether I'm running in the open (should be good GPS reception) or in the woods. I suspect that this means the Garmin is slightly more accurate because I assume that if the GPS signal drops out, then is picked back up, the software fills in the missing bit with a straight line, which might be shorter than actual path. But I don't actually know that, and the difference is pretty small anyway. To me, there are bigger issues with the ANW.
- The heart rates reported by the ANW and the G620 sometimes match exactly, but other times do not. I haven't yet compared them systematically. I use a chest strap with the G620, so my guess is that it is more accurate.
- Remember that all Apple Watch displays, including the ANW, are "off" until you tap the screen or turn/raise your arm. In practice, this means that checking your ANW requires taking your eyes off where you are going for longer than with a regular watch. Whether this bugs you, and to what degree, is probably a matter of individual preference, but it's something to consider and (of possible) try out with an Apple Watch, even just standing in the store.
- I have not found a running app for the ANW that I like, and I think I've now tried most of them except Strava (built in Apple Workout, Runtastic, Nike Run Club, iSmoothRun, MapMyRun, RunKeeper). Obviously, this is partly a matter of personal taste, and I don't have the time now to review each app individually. But I assume you have an iPhone (need one for an Apple Watch to work), so you can go on the App Store and look at what running apps also offer watch apps and check out the screen shots. My big issue is that the interface during a run almost always highlights total elapsed time, which I don't really care about. With the G620, I can configure the displays so that they show me the info I'm interested in (pace, heart rate, distance). Not so with ANW apps, at least that I've found. Also, many of those apps charge a monthly fee.
- Beyond the "in run" interface, the Nike Run Club app that is featured on the ANW is a total dumpster fire. Good luck pulling up completed runs, for example.
- The "auto pause" function for at least some ANW running apps seems to be based on arm movement. It's surprisingly awkward to keep one arm still while waiting at a stoplight. Of course, you can use the manual pause function, but that requires one or more screen presses on most apps (which, in turn, means that you have to look at the screen rather than feeling for a button). To me, the mechanical button press to pause on the G620 is way better, though I'm also more used to that.
- Overall, I'd say the Garmin versus Apple Watch question is case of a specialized tool versus a general tool. For running, I think most people would find a Garmin to be better. But a Garmin obviously can't do texts, phone calls, emails, or hail you an Uber when you bonk, all of which an Apple Watch can, so it is the better general tool. Of course, to do all that stuff with an Apple Watch, you also need to have your iPhone with you (and have cell reception), so the Apple Watch isn't really adding any functionality ... it just makes it more accessible by putting some of it on your wrist.
Hope this helps.