Anthropic use the names Haiku/Sonnet/Opus for the small/medium/large versions of each generation of their models, so within-generation that is also their performance (& cost) order. Evidentially Sonnet 3.5 outperforms Opus 3.0 on at least some tasks, but that is not a same-generation comparison.

I'm wondering at this point if they are going to release Opus 3.5 at all, or maybe skip it and go straight to 4.0. It's possible that Haiku 3.5 is a distillation of Opus 3.5.