It's the second case that's more important: if you find a bug in production, and the test that covers that functionality is green, then there's a problem with y

It's the second case that's more important: if you find a bug in production, and the test that covers that functionality is green, then there's a problem with your test suite. Truth is, there's no need for an either/or holy war about this. The two styles are easily mixable: property based testing for broad coverage, specific runs for specific coverage.