depends on your budget and usage bah, but I think money is not an issue for your case.

1. Battery Ultra2 of course is better than S9
2. Casing, Ultra2 is titanium while S9 is either alu/stainless steel
3. If you're diver, ultra2 is for you. 100M vs S9 which is only 50M
4. Screen bigger on U2, I think size is an important point to take note. Some prefer smaller
5. U2 got siren features, I think is for those extreme sport and you need it for emergency
6. GPS more accurate on U2

Other than that, is about the same. another is S9 is 2k nits while U2 is 3k which is brighter, so you can look at  your watch at an extreme sunlight or deeper in the sea I guess

Picture this: I’m making the arduous journey back from Trader Joes to my apartment with a heavy tote of groceries when my mom calls out of the blue. You always have to answer the phone when mom calls, right? But with my right hand holding my bag firmly and my iPhone stowed in my purse, I’m torn between letting the call buzz out (possibly sending my mom into a panic) or attempting to awkwardly tap the answer button with my nose.

Except now, with either the new Apple Watch Series 9 or Apple Watch Ultra 2, I can simply tap my free index finger and thumb on my watch hand twice to answer the call, hang up the call, or execute a slew of additional controls.

I had the chance to try double tap for Apple Watch myself after the Apple event on September 12. Though the feature won’t roll out to devices until October, there were some units with double tap available to try. And while I wasn’t in a situation such as I described above, I got a taste of what the most convenient Apple Watch experience yet feels like.

Double tap works with your wrist raised and your Apple Watch Series 9 or Apple Watch Ultra 2 awaiting the gesture in the background. Yes, although the next-gen Apple Watch devices don’t have a new optical heart sensor reading the minor wrist movements and changes in blood flow, only the upgraded S9 SiP supports the gesture’s algorithm.

There are too many applications of double tap to name, but I’ll walk you through the ones I tried for myself beyond answering phone calls. First and foremost, double tap opens the Smart Stack, the standout watchOS 10 update that curates a dynamic tile carousel on your wrist. When I continued the double tap gesture, the demo watch cycled through a collection of calendar, timer and music tiles.

When I opened the timer app and started a 10-second countdown, I could use double tap to pause, restart, and dismiss the timer. I imagine this will be helpful when I’m cooking and managing multiple timers while running a chicken dredge station.

Finally, I saw how double tap totally improves the Camera Remote app. I use Camera Remote to remotely facilitate iPhone capture content quite often, though it can be cumbersome depending on the type of photos or videos I’d like to take. Sometimes I’m intentionally holding an object in my non-Apple Watch hand, making it tricky to tap the capture button. Double tap solves this problem.

Double tap can also pause and play music, trigger talk-to-type for text messages, snooze alarms and more. I didn’t try these functions myself, but all in all, this suggests that double tap will permeate many parts of the Apple Watch user experience. I imagine it’ll take some getting used to, but as second-nature as it is to raise my wrist to wake the wrist, I expect double tap will become integral to navigating the watch.

That said, the large-scale double tap adoption will take time. Since earlier-generation Apple Watch devices don’t support double tap, it’ll probably be a few upgrade cycles until a majority of users are able to access the feature. I don’t think double tap is enough to encourage an update from the Apple Watch Series 8, but we’ll have to wait to see how it's received once it's working next month.

Source: I just tried the Apple Watch 9 double tap feature — and I'm convinced it's the future.

Dual GPS only on Ultra & Ultra2 for now.

Normal SE/S series used L1, while Ultra is L1 & L5
L5 is more accurate and combine with L1, of course it is better than non-ultra as it will lock the location faster and more efficiency