For quick sketches and brainstorming sessions, especially if you appreciate a hand-drawn aesthetic, Excalidraw is your best choice. It's ideal for developers sketching architecture or anyone who needs to visualize ideas rapidly without the need for polished finishes. If your focus is on immediate, low-fidelity output and keeping your data local, Excalidraw fits perfectly.
However, if you need a robust system for personal organization and project management, opt for Obsidian. Its extensive customization options and local storage provide greater privacy and flexibility, making it well-suited for building your knowledge base. Just keep in mind that there's a learning curve, and if real-time collaboration is essential or you prefer seamless cloud integration, Obsidian may not meet your needs.