There is no difference between metaphoric and metaphorical, but the latter is more common than the former. Unlike similar word pairs such as historic–historical and economic–economical, metaphoric and metaphorical have not undergone differentiation. They are more like the undifferentiated ironic–ironical, except that in this case the –ic word is the unnecessary variant (whereas ironical is considered an unnecessary variant of ironic).
Metaphoric will continue to appear as long as long as the –ic/-ical suffixes remain tricky (i.e., probably forever), and some writers will always favor it for its brevity, but metaphorical prevails for now.
This ngram graphs the use of metaphoric and metaphorical in English-language books published in the 20th century.