Which one should you use?
Price is only half the decision: run both on a sample of your own prompts and compare quality, speed and failure rate. If DeepSeek V4 Pro 0813 is good enough for a task, it saves $5.35 per thousand chat messages. A common setup is to route easy requests to the cheaper model and keep GPT-6.1 Sol Pro for the hard ones.
Both are available behind one OpenAI-compatible key through OpenRouter or a reseller, so switching is a one-line change. See where credits cost least today and how to cut the token bill.