If you're not a cyclist then it doesn't matter but there's definitely lots of stuff the Garmin can do that the Apple Watch can't.
I have 8.5 inch wrists, and even though that's relatively chunky, the Garmin looks absolutely ridiculous.
The thing about fitness and sleep tracking is it's only useful if you're wearing the device pretty much all the time, particularly for things like daily calorie burn.
Whereas I found that there are so many contexts where it either wasn't appropriate, or I didn't want to, wear a gigantic activity watch. I slugged it out for a few months, but in the end gave up on it because it wasn't really practical or stylish.
The maps are something of a wash as well, because the screen is so tiny.
My other arguments stand, to the point where I wouldn't even recommend buying one of the smaller ones unless you really need the water resistance. I imagine even the smaller models still have better battery life than the Apple Watch, so that's another consideration.
I have a Garmin Fenix 5X and it does not make the Apple Watch look like a toy. The Apple Watch integration is seamless, the Garmin Connect integration is a joke.
Moreover the Fenix has a clunky user interface, and is bulky and ugly.
The Apple Watch absolutely trounces the Garmin in every aspect, functional and aesthetic, except for two:
1) Water resistance: it's hard to argue with the Garmin's 100M water resistance.
2) Battery life, but it only does better here by being massive (ref. previous comment about bulkiness).
Seriously, unless you're spending a lot of time in the water, and if you're able to put the watch on charge every night, buy the Apple.