BUT! Garmin is much at creating accurate GPS tracks.
Blue line is Garmin, Orange is Apple Watch.

The Apple Watch «only» has a single band GPS, it does a good job, but it smooths out bends and turns. Not a big issue, but something to keep in mind.

The Garmin does a perfect job, it gets every turn and switchback!

I know mountain biking and especially downhill is very hard for wrist based monitors. There is a lot of movement, a lot of tension in the wrists and insane amounts of vibrations.

My best guess is that it’s the weight of the Garmin watch does undoes it. It’s a lot more mass and inertia that disturbs the readings.

The series 10 is much lighter, and slimmer, and that makes it easier for it to stick to the skin, and get better readings.

I use sports loops for both watches, for a tight fit