The verdict
If you collect leads, applications, feedback or registrations from people, pick Fillout. If you ship code with more than one contributor, human or agent, pick GitHub. Most teams that write code end up with GitHub regardless.
Do not use either for the other's job. GitHub has no form builder for customers, and Fillout has no place for code history. If you only need a backup of one folder, GitHub is more than you need. If you only need one simple form, Fillout is more than you need.