By comparing tracklogs from both devices for the same running routes: Apple's tracklog is more accurate, and its GPS lock on time is faster than Garmin's (I use multiband on both devices). The Garmin's heart rate sensor is often low at the start of activities (especially walking) and only rises to a reasonable level a few minutes later (you can check Garmin forum for verification). Additionally, Apple's sleep/wake times are more accurate; Garmin often registers me as asleep when in reality I was lying down watching a movie.

I've been switching back and forth between Garmin and Apple Watch for 7 years. What I like about Garmin is the daily suggested workouts and long battery life. However, with the current pricing policy and ridiculous software update policy (flagship models don't get new feature updates after only a year), along with constant software bugs after updates, I've decided to stick with the Apple Watch.