Both would be good. My experience working on open source projects involved months of working with the maintainers and community to reach consensus on designs an

Both would be good. My experience working on open source projects involved months of working with the maintainers and community to reach consensus on designs and implementation details. That is a very useful skill, but you aren't going to get that on a pet project. On a pet project however, you'll get to see what they can do when they move fast which will give you better insight into their engineering capabilities.