I'm a watch collector, and own a ton of watches with both glass and sapphire crystals. Long story short, the glass crystals accumulate dings, the sapphire ones are pristine after years of wear. Sapphire is inarguably the better material overall.
With this in mind, I bought the S0 SS/Sapphire Apple watch when they first came out, and while the crystal was pristine when I replaced it with a Series 3, I didn't feel like the upgrade paid off well over the 2-3 years I might wear an Apple Watch, vs. a much longer ownership life of a traditional wristwatch.
So, for my Series 3 and current Series 5, I've gone with the base aluminum/ion-x versions, and so far am scratch free. I'm sure that I'll eventually put a ding in one with so much exposed crystal area, but it hasn't happened so far.
They've held up considerably better than the 'Hardlex' mineral glass of my Seiko dive watches, which is supposedly above-average for the category, so I'm relatively clear that the ion-x glass is meaningfully superior to at least that one example, if that's helpful.
TL;DR - Sapphire is way better, but the upgrade cost isn't worth it for me.