Ultra has almost the double physical battery and may have some exclusive optimization to GPS and HR that may not come to other watches when released later this year.
The Fenix 7 supports dual frequency GPS. That and the battery life improvements seem to be the significant differentiators between the Fenix 6 and 7. I’ve gotten very used to the Fenix UI when backpacking and have it basically committed to muscle memory at this point. For that reason I’d much prefer to upgrade my Fenix every other generation than move to the Apple Watch. Also, if you’re hiking with some sunlight, the solar charging feature of the solar editions of Garmin watches are quite nice. I’ve squeezed a few extra days of the watch before thanks to solar, and that’s been invaluable.