There are indeed a few (and rather uncommon) licenses that the FSF consider
free but the OSI does not consider open source and vice versa. I do not think
it is worth searching for practical difference between the two terms. They
are essentially the same (Bruce Perens, who wrote the open source definition,
took the FSF definition as a base and tried to make it more "concrete").
However, there is a philosophical difference that is very significant! When
Eric Raymond coined the term "open source" in 1998, he did so to precisely
avoid talking about freedoms and, instead, focus on practical advantages such
as a better quality, a better security, etc. Those practical advantages are
not clear at all (most "open source" projects have one single developer, the
technically best alternative often is proprietary, etc.). Today, "open
source" proponents still do not grant much attention to user freedoms. For
instance, they do not see tivoization as a problem (nonfree executables made
from source code that is free).
On the contrary, the free software movement, lead by rms since 1983, mainly
is an ethical/social/political project. It aims at freeing computer users,
i.e., at making them in control of their own computing. I consider it to be a
more fundamental goal (control one's own computing ought to be a fundamental
right) and a more compelling argument for "sticking to free software" (as the
title of this thread says): free software always is better than proprietary
software because it grants the users essential freedoms proprietary software
developers deny.
In the end, "Open Source misses the point of Free Software".
