Verdict
Built from the measurements below
- Accuracy is too close to call: GPT-5.3-Codex scored 82.5% and Muse Spark Open 77.5% on the same 80 questions, a gap within what chance alone could produce.
- Muse Spark Open is 2.3× cheaper per question as billed ($0.0045 vs $0.01), reasoning tokens included.
- Muse Spark Open generates text 2.3× faster: 220 tokens per second against 95.
- Muse Spark Open starts answering sooner: 0.5 s to the first token, against 1.4 s for GPT-5.3-Codex.
- Muse Spark Open accepts more context: 1M tokens against 400k.
Which should you choose?
Choose Muse Spark Open for the lower cost per correct answer ($0.0058 against $0.013), faster streaming, a quicker first token, open weights you can run yourself, a 1M-token context window and more hosts to choose from on OpenRouter (6 against 2). The widest subject gap is in science: GPT-5.3-Codex answered 28 of 30 correctly, Muse Spark Open 25 of 30. With so few questions per subject, treat this as a lead to test on your own prompts, not a result.
Accuracy gap +5.0 points (z = 0.79). We call a winner only when z is at least 1.96, the usual 95% threshold. Speed, latency and price need a 10% margin.