Verdict
Built from the measurements below
- Accuracy is too close to call: GPT-5.3-Codex scored 82.5% and o3 72.5% on the same 80 questions, a gap within what chance alone could produce.
- o3 is 1.2× cheaper per question as billed ($0.0088 vs $0.01), reasoning tokens included.
- o3 generates text 1.7× faster: 163 tokens per second against 95.
- o3 starts answering sooner: 1.2 s to the first token, against 1.4 s for GPT-5.3-Codex.
- GPT-5.3-Codex accepts more context: 400k tokens against 200k.
Which should you choose?
Choose GPT-5.3-Codex for a 400k-token context window. Choose o3 for faster streaming and a quicker first token. The widest subject gap is in engineering: GPT-5.3-Codex answered 11 of 15 correctly, o3 8 of 15. With so few questions per subject, treat this as a lead to test on your own prompts, not a result.
Accuracy gap +10.0 points (z = 1.51). We call a winner only when z is at least 1.96, the usual 95% threshold. Speed, latency and price need a 10% margin.