After conducting extensive head-to-head testing between Claude Sonnet 4 and Gemini 2.5 Pro Preview using identical coding challenges, I've uncovered significant performance disparities that every developer should understand. My findings reveal critical differences in execution speed, cost efficiency, and most importantly, the ability to follow instructions precisely. I designed my comparison around real-world coding scenarios that test both models' capabilities in practical development contexts. While Gemini 2.5 Pro Preview offers compelling cost advantages and creative capabilities, its tendency toward scope expansion makes it better suited for exploratory rather than production development contexts. Gemini 2.5 Pro Preview's creative capabilities and extensive context window make it valuable for specific use cases, but its tendency toward scope expansion requires careful consideration in production workflows where predictability and constraint adherence are paramount.