Verdict
Built from the measurements below
- Accuracy is too close to call: Claude Opus 5.5 scored 85.0% and Claude Sonnet 5.5 78.8% on the same 80 questions, a gap within what chance alone could produce.
- Claude Sonnet 5.5 is 2.7× cheaper per question as billed ($0.0054 vs $0.015), reasoning tokens included.
- Output speed is similar (129 vs 126 tokens per second).
- Claude Sonnet 5.5 starts answering sooner: 0.7 s to the first token, against 2.3 s for Claude Opus 5.5.
Which should you choose?
Choose Claude Sonnet 5.5 for the lower cost per correct answer ($0.0069 against $0.017) and a quicker first token. The widest subject gap is in law: Claude Opus 5.5 answered 5 of 6 correctly, Claude Sonnet 5.5 3 of 6. With so few questions per subject, treat this as a lead to test on your own prompts, not a result.
Accuracy gap +6.3 points (z = 1.03). We call a winner only when z is at least 1.96, the usual 95% threshold. Speed, latency and price need a 10% margin.