Verdict
Built from the measurements below
- Accuracy is too close to call: Gemini 3.1 Pro Preview scored 77.5% and Gemini 3.8 Flash 82.5% on the same 80 questions, a gap within what chance alone could produce.
- Gemini 3.8 Flash is 4.6× cheaper per question as billed ($0.0037 vs $0.017), reasoning tokens included.
- Gemini 3.8 Flash generates text 1.1× faster: 142 tokens per second against 124.
Which should you choose?
Choose Gemini 3.8 Flash for the lower cost per correct answer ($0.0045 against $0.022) and faster streaming. The widest subject gap is in law: Gemini 3.8 Flash answered 6 of 6 correctly, Gemini 3.1 Pro Preview 3 of 6. 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.