Although I agree that both terms are not exactly equal as they emphasize different aspects, those differences are totally irrelevant to the argument at hand:
If you advertise your project by stating you are convinced of the "power of Open Source", but don't make use of that "power", how much credible is that?
No matter if you find this argument convincing or not, it has nothing to do with the difference between Open Source and Free Software. It's just about adverticing a proprietary project with "power of Open Source" versus advertising it with, say, "power of Vim".
