Which is more cost-efficient on simple tasks: Sonnet 5 max or Opus 5 low effort?

Neither is actually the recommended choice for this scenario - for a short, simple task, the better move on either model is using a low effort setting on the model that's already well suited to simple tasks, rather than maxing out effort on the smaller model or reaching for the larger, pricier model at all.

Sonnet 5 at max effort specifically is flagged in reporting as a poor value pattern: it can roughly double cost relative to lower effort settings without a correspondingly large quality improvement, which is a bad trade specifically on a task that didn't need that much reasoning depth in the first place.

Opus 5 at low effort is a more defensible pattern in general, since Opus 5's effort dial is specifically designed to let it stay cheaper and faster on easy requests without switching models entirely, but for a genuinely short, simple task, this still isn't obviously better than simply using Sonnet 5 at an appropriately low effort level, given Sonnet 5's lower baseline per-token price.

The more useful framing than picking one of these two specific configurations is to match effort to actual task difficulty on whichever model is already the right tier for the task category, and to treat both maxing effort on the smaller model and using the larger, pricier model for a task that didn't need it as patterns worth avoiding, rather than as a genuine two-option tradeoff.