Clang has a permissive license, GCC is (of course) GPL.

Which one is best for open source is debatable.

Permissive licenses make it easier for companies to make proprietary software, or even a closed source version of the original project.

Copyleft licenses (like GPL) are intended to promote free/open source software but they can be a legal headache, which can make users favor proprietary solutions.

On HN, I think that people tend to prefer permissive licenses (but complain when large companies "steal" their work, go figure...).