Verdict
Built from the measurements below
- Accuracy is too close to call: GPT-5.6 Terra scored 75.0% and GPT-6 Sol 73.8% on the same 80 questions, a gap within what chance alone could produce.
- GPT-6 Sol is 2.7× cheaper per question as billed ($0.0024 vs $0.0063), reasoning tokens included.
- Output speed is similar (101 vs 92 tokens per second).
- GPT-6 Sol starts answering sooner: 0.9 s to the first token, against 1.1 s for GPT-5.6 Terra.
Which should you choose?
Choose GPT-6 Sol for the lower cost per correct answer ($0.0032 against $0.0084), a quicker first token and shorter outputs (about 174 tokens per question against 472). The widest subject gap is in engineering: GPT-5.6 Terra answered 11 of 15 correctly, GPT-6 Sol 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 +1.2 points (z = 0.18). We call a winner only when z is at least 1.96, the usual 95% threshold. Speed, latency and price need a 10% margin.