TL;DR
Use Claude 3.5 Sonnet as your default for reasoning, long context, writing, and agent backbones. Use GPT-4o when you need native function calling reliability, the broader OpenAI ecosystem (Whisper, Realtime API, DALL-E), or sub-cent token economics with GPT-4o-mini. Most production stacks use both.
Where Claude wins
- Long-form writing — produces fewer "AI tells" and tighter prose
- Reasoning over long context — handles 200K+ tokens better, less mid-context degradation
- Code generation — particularly for full file edits and refactors
- Following nuanced instructions — adheres to "do this, not that" more reliably
- Conversation tone — feels less robotic in customer-facing chat
Where ChatGPT (GPT-4o) wins
- Native function calling — slightly more reliable in production agent loops
- Realtime voice (sub-300ms latency) — Realtime API is currently in a class of its own
- Cost at volume — GPT-4o-mini at $0.15/M input is the price floor
- Image generation — DALL-E + GPT integration tightly bundled
- Structured outputs guarantee — JSON schema enforcement is rock-solid
- Speed on short tasks — usually ~30% faster on sub-1K token responses
Where it doesn't matter
- Most chatbot Q&A — both will hit 90%+ accuracy with proper retrieval
- Email drafting / first-pass copywriting
- Translation
- Sentiment analysis / classification
What we actually run in production
- Default reasoning model: Claude 3.5 Sonnet
- High-volume classification: GPT-4o-mini
- Voice agents: GPT-4o Realtime
- Deep reasoning / planning: Claude 4 Opus or o3 (depends on cost budget)
- Embeddings: OpenAI text-embedding-3-large (Cohere's a close second)
Full benchmark with prices, latency, and per-task scores: ChatGPT vs Claude vs Gemini 2026.