If you have an iPhone, it's a good move.

There are a few small things Garmin watches are better at, but there's a massive list of things Apple Watches do better than Garmin ones. It's not even close.

I'm pretty sure the Series 9 is mostly unchanged because there's basically nothing left Apple can do to improve the hardware... it's just a waiting game for batteries that hold more power - which will allow more features.

On battery life... Garmin advertises "8 days" for the Vivoactive 4 but that's only if you don't use certain features. It's exactly the same with an Apple Watch... real world battery life depends what is active, how many notifications you receive, how often you check the time, how often you are out of bluetooth range of your phone or a wifi network, etc etc.

Generally because Apple watches can do more than Garmin watches, you're likely to get less real world battery life. But it's easy enough to charge them once a day (20 minutes or so on the charger is enough. Doesn't have to be overnight).

The lowest power mode an Apple Watch can run in (it will only tell you the time) will last a full month on a charge.