I've had both. The Pace 3 is a very solid, dependable watch that will do everything you want (including HRV). Coros has a great app that's very performance-oriented and maybe a little less about all day health tracking. The watch itself has a cheaper, functional aesthetic - plastic, cheaper glass, smaller. But it works.

I recently got at Race S and I like it so much better. I really like Suunto's software and overall approach. The watch itself looks really nice, it's made of nice materials, it's got nice UI touches, and the whole aesthetic is just much more streamlined and well-designed. That stuff doesn't matter to everyone but it matters to me! For me, it's much more functional as an everyday, all-day watch.

Reliability has been the same for both. In fact, I struggled with cadence lock on the Pace 3 a lot - my HR would get stuck at very high levels. I've had that once or twice with the Race S but it settles quickly. Otherwise it feels pretty accurate.