DeepSeek R1 outperformed OpenAI o1-mini in detecting subtle software bugs in real-world programs, with a stronger planning step and deeper reasoning loop contributing to its success. The evaluation dataset consisted of 210 programs with small, difficult-to-catch, and realistic bugs introduced by the author. DeepSeek found over two times more bugs than o1-mini, with notable strengths in TypeScript and Rust where reasoning through asynchronous logic and error handling is crucial. In contrast, o1-mini excelled in simple, pattern-based bugs but struggled with deeper logic or asynchronous issues. The study highlights the importance of reasoning capabilities in AI-assisted bug detection, particularly in concurrency-heavy or async-first environments.