I have both a SS S9 and a Alum S9. I play pickleball and cycle, so I don't want to wear my nice SS watch doing these activities that could really damage them. Yes, I occasionally hit the deck, or run into a fence. LOL So, I use a cheaper Alum watch that I usually get at a discount for the rough stuff and for sleeping.
However the SS is great for everything else. For most folks, if you have to pick just one of the watches, I'd go for the SS. The crystal stays cleaner imo, and is more durable, and it's a better looking watch. And, it comes with cellular which you may decide to use one day.
I am trading in the Alum S9, for the Alum Jetback S10. Bestbuy gave me $300 for it which is about what I originally paid, and I got to keep the nike band that I like. 👍🏼 IF it weren't for the great trade-in amount, I would have kept the Alum S9. The S9 is still a great watch.