If you prioritize an open-source environment and need a design tool that respects privacy, Penpot is your best choice. It allows for self-hosting and offers real-time collaboration that benefits both designers and developers. However, if you're looking for a polished, user-friendly experience with a vibrant plugin ecosystem, you'll find Penpot lacking, particularly if advanced features of tools like Figma are essential for your work.
On the other hand, if you require an infinite canvas for product development and appreciate the ability to create custom applications, tldraw stands out. Its polished interface and special features like the "make real" function cater well to developers and product teams. However, be aware that it is source-available rather than fully open-source, which may be a dealbreaker if your project requires true open-source licensing. Avoid it if you seek a simple, free sketchpad, as tldraw's premium aspects may not justify the cost for basic needs.