It's a trade-off. The sapphire glass is far more scratch-resistant, but less shatter-resistant. It's hard to predict which is more likely - if you're worried, get Apple Care. It's more expensive than last year but still not a bad deal if you think you're going to crash the watch into things.
I bought a Series 3 Nike+ (ion glass) last year. I got Apple Care because I do tend to bang into things. One year later, no scratches and no shattered displays. I just bought a Series 4 - I did not get Apple Care this time. The Watch turns out to be a lot more durable than I had thought. YMMV, though.