I think that synonyms typically overlap, i.e. cover part of the same
semantic territory. So while they do not coincide in their whole
(range of) meaning, they do coincide in much of it. It seems to me that
in those areas where they do coincide, they can often be used
interchangeably (from a semantic perspective) and in those cases
stylistic considerations may well come to bear on the choice between the
two (or more). Of course this particular passage practically invites
one to speculate about differences in meaning because of Jesus'
*apparent* insistence on using AGAPAW and Peter's *apparent* insistence
on using FILW. But of course such speculation cannot be based on this
passage alone, as the numerous passages cited by other posters amply
demonstrate.
Ron Ross