After some time replacing gemini 3 flash preview with deepseek v4 flash for a chat model, the biggest difference is the auto reasoning effort. Gemini flash is super fast and perfect for a chat model. But when I need some thought experiments with a handful of constraints, it struggles a bit and I switch to sonnet. But with deepseek v4 flash, it can do long complex reasoning and it gets things often right. Generating a lot of reasoning tokens means that it takes a lot of time of course. But I am happy to find a cheaper model and excited to try something other than gemini flash. Gemini flash has been so good that I was locked on it for a while.