If we were to only compare survival after a fixed number of drives failed vs. storage efficiency, RAIDzN should always come out ahead in any configuration - with mirrors you can get unlucky drives choice fail, with RAIDzN any choice is a good choice. Only way to have RAIDz fail sooner than mirror is to have a comparatively less redundant setup (your choice of N and K).
Realistically though, RAIDz recovery is longer and more stressful, so more of your drives can fail in the critical period, and, assuming you have backups, your storage is there for for usability - mirroring gives you a performant usable system during a fast recovery for the price of a small chance of complete data loss (but you have backups?) vs RAIDz that gives you long recovery pains on a degraded system, but I expect a smaller chance of data loss on a lightly loaded system.
Realistically though, RAIDz recovery is longer and more stressful, so more of your drives can fail in the critical period, and, assuming you have backups, your storage is there for for usability - mirroring gives you a performant usable system during a fast recovery for the price of a small chance of complete data loss (but you have backups?) vs RAIDz that gives you long recovery pains on a degraded system, but I expect a smaller chance of data loss on a lightly loaded system.