Those of us in the free/libre and open source software (FLOSS) community know the routine by now. Despite the fact that "free software" and "open source" refer to the same software and the same communities, supporters of "free software" like the FSF would have us advocate for FLOSS by talking about users' rights to use, modify, share, and cooperate; open source supporters like the Open Source Initiative would have us advocate for software by talking about how securing these rights produces software with "better quality, higher reliability, more flexibility [and] lower cost."
One reason I tend to stay away from "open source" claims in my own advocacy is that I'm worried by the way that these arguments rely on a set of often dubious empirical claims of superiority. Free software, on the other hand, can be seen as statement of principles. Regardless of whether we say "free software" or "open source," I've found that a focus on principled statements is both more robust against counter-arguments and does a better job of describing the motivations of most contributors.