Verdict
Built from the measurements below
- Accuracy is too close to call: Claude Fable 5.1 scored 81.2% and Gemini 4 Argon 84.8% on the same 80 questions, a gap within what chance alone could produce.
- Gemini 4 Argon is 2.6× cheaper per question as billed ($0.0063 vs $0.016), reasoning tokens included.
- Output speed is similar (70 vs 69 tokens per second).
- Claude Fable 5.1 starts answering sooner: 2.1 s to the first token, against 3.3 s for Gemini 4 Argon.
Which should you choose?
Choose Claude Fable 5.1 for a quicker first token, shorter outputs (about 238 tokens per question against 558) and more hosts to choose from on OpenRouter (4 against 2). Choose Gemini 4 Argon for the lower cost per correct answer ($0.0074 against $0.02).
Accuracy gap -3.6 points (z = -0.60). We call a winner only when z is at least 1.96, the usual 95% threshold. Speed, latency and price need a 10% margin.