While the terms are usually used in reference to identical groups of software, the open source and free software movements have different values, philosophies and histories.
Open Source is a development methodology; Free Software is a social movement:
Free Software is primarily concerned with freedom while open source is primarily concerned with better software.
Eric Raymond, the philosophical lead of the Open Source movement, claims that suits run as soon as they hear the word “freedom” they run.
Not only am I not afraid to discuss the idea of software freedom openly, I think it's essential to highlight freedom.
...especially when I'm talking to non-profit organizations, many of which are driven by similar philosophical concepts, virtues, and ideals.