Comment by jraph

Free software licenses and open source licenses are almost the same set, and the difference between the two is not copyleft: the GPL is OSI-approved. The difference is in the spirit, focus, philosophy: free software is concerned about user rights.

And if you care about user rights, use a copyleft license to prevent someone from building a proprietary derivative, even if you are not turning it into your day job. Especially if you are not turning it into your day job, a copyfree license is more likely to atract corporate.