This is a hard problem for me as well. Right now I've just been using the best model available (like Opus, or GPT 5.5, or Gemin Pro) but it's not ideal. My problem is anytime I step down the results are subtlely worse and sometimes I don't notice immediately depending on what I'm doing.

As far as Opus vs. GPT 5.5 etc, I generally decide with:

1. Code? -> Opus

2. Docs? -> GPT

3. Real-time or recent information needed? -> Gemini

It's far from perfect though. Would love to hear others thoughts.

Opus eats tokens so fast so I try to minimize it but compared to Sonnet I definitely see fewer issues in my larger projects. Sonnet has gone off the rails a few times.