Verdict
Built from the measurements below
- Accuracy is too close to call: Claude Sonnet 5.5 scored 78.8% and GPT-5.3-Codex 82.5% on the same 80 questions, a gap within what chance alone could produce.
- Claude Sonnet 5.5 is 1.9× cheaper per question as billed ($0.0054 vs $0.01), reasoning tokens included.
- Claude Sonnet 5.5 generates text 1.3× faster: 126 tokens per second against 95.
- Claude Sonnet 5.5 starts answering sooner: 0.7 s to the first token, against 1.4 s for GPT-5.3-Codex.
- Claude Sonnet 5.5 accepts more context: 1M tokens against 400k.
Which should you choose?
Choose Claude Sonnet 5.5 for the lower cost per correct answer ($0.0069 against $0.013), faster streaming, a quicker first token, shorter outputs (about 456 tokens per question against 699), a 1M-token context window and more hosts to choose from on OpenRouter (5 against 2).
Accuracy gap -3.7 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.