This research paper compares and contrasts two types of software agents powered by large language models (LLMs): API-based agents and GUI-based agents. API agents interact with software through programmatic interfaces, offering efficiency and reliability, while GUI agents mimic human interaction by operating through graphical user interfaces, providing flexibility and broader applicability. The paper analyzes the differences in their architecture, development, and user interaction, also exploring emerging hybrid approaches that combine the strengths of both. Ultimately, it offers guidance on selecting the most suitable agent type based on specific application scenarios and anticipates future trends in LLM-driven automation.