Today, just for the heck of it, I decided to put my Apple Watch, Series 3 Stainless, up against my Fenix 5x and see how the GPS tracks compared. They were both on the same wrist, about an inch apart, and worn at the same time. This "should" make the tracks fairly equal for comparison purposes. The F5x is using a fully calibrated footpod, (MiCoach Bluetooth), while the Apple watch is using it's own semi-calibrated distance algorhythms. Both used the internal HRM's of the watches. I then exported both files, and imported them into MyGPSFiles, to see the comparison. It sould also be noted that the actual measured elevation at the starting point is 1432 meters. I used GPS Only on the F5x, as I've heard that GLONASS and Galileo leave a bit to be desired. I have no idea what GPS array the AW was using. Here is the file comparing the two tracks in MyGPSFile.
http://mygpsfiles.com/app/#MgcgZF58
Come to your own conclusions. :-)