Verdict
Built from the measurements below
- Accuracy is too close to call: Claude Opus 5.5 scored 85.0% and Gemini 3.1 Pro Preview 77.5% on the same 80 questions, a gap within what chance alone could produce.
- Claude Opus 5.5 is 1.2× cheaper per question as billed ($0.015 vs $0.017), reasoning tokens included.
- Output speed is similar (129 vs 124 tokens per second).
- Claude Opus 5.5 starts answering sooner: 2.3 s to the first token, against 3.4 s for Gemini 3.1 Pro Preview.
Which should you choose?
Choose Claude Opus 5.5 for the lower cost per correct answer ($0.017 against $0.022), a quicker first token, shorter outputs (about 645 tokens per question against 1,382) and more hosts to choose from on OpenRouter (5 against 2). The widest subject gap is in law: Claude Opus 5.5 answered 5 of 6 correctly, Gemini 3.1 Pro Preview 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 +7.5 points (z = 1.22). We call a winner only when z is at least 1.96, the usual 95% threshold. Speed, latency and price need a 10% margin.