I’ve had both and currently using the AW Ultra. As other have mentioned, garmin excels in fitness tracking but Apple is very close.
Where Apple falls short compared to Garmin is battery life. Most of apples battery life is consumed by the display (which is fantastic), background apps and connectivity.
Personally, the trade of shorter battery life for conveniences such as; calls/texting, Siri, music etc etc is worth it to me. If you are a long distance runner or anything where extended fitness/gps tracking without being able to recharge is important, then Garmin is the way to go. If you do routine workouts and are able to charge your watch daily then Apple is a very good option.
From a hardware perspective you don’t need the ultra to use the athletics app. The sensors on the S9 will perform exactly the same.
As mentioned, the advantage of the ultra is the battery life, durability and larger screen which can all be helpful for certain activities.