Verdict
Built from the measurements below
- Accuracy is too close to call: Claude Fable 5.1 scored 81.2% and Muse Spark Open 77.5% on the same 80 questions, a gap within what chance alone could produce.
- Muse Spark Open is 3.6× cheaper per question as billed ($0.0045 vs $0.016), reasoning tokens included.
- Muse Spark Open generates text 3.1× faster: 220 tokens per second against 70.
- Muse Spark Open starts answering sooner: 0.5 s to the first token, against 2.1 s for Claude Fable 5.1.
Which should you choose?
Choose Claude Fable 5.1 for shorter outputs (about 238 tokens per question against 964). Choose Muse Spark Open for the lower cost per correct answer ($0.0058 against $0.02), faster streaming, a quicker first token, open weights you can run yourself and more hosts to choose from on OpenRouter (6 against 4).
Accuracy gap +3.7 points (z = 0.59). We call a winner only when z is at least 1.96, the usual 95% threshold. Speed, latency and price need a 10% margin.