That's a really good question! In practice the difference is not
that big but the main intention is like this:
debian-med@lists.debian.org is intended for users who wo are using
the software we are assembling. The discussion should not be so
highly technicall (but in fact we well not "bann" any discussion
about packaging here.)
debian-med-packaging@lists.alioth.debian.org is the "Maintainer"
of our packages and the discussion should be focussed on technical
details which might be not interesting for the user of the
packages.
In the end there is no strict borderline between both lists and you are
free to decide what list to use - I guess nobody will blame you to have
used the wrong list.