Honestly you would be best off taking a year to reapply directly to medicine, graduate entry medicine is considerably more competitive than standard entry medicine, and even more considerably competitive for international fee paying students. Even for domestic students (for whom there are many more places) graduate entry medicine is not recommended as an aim for school leavers, and they would be best advised to take a gap year and if needed resit exams. As an international student the odds are astronomically against you for graduate entry medicine.
In any event, if you do insist on such a route, neither is better than the other for the purposes of GEM admissions as where you studied is not an important factor in applying to GEM. I don't know if there's a significant difference between them academically for that subject, so I suspect it would really depend on your personal preferences for a uni, how you like each city compared to the other (Cardiff is I think a fair bit bigger than Exeter), costs of living, ease of getting to the uni and transportation links (consider where nearby major international airports are and connections from there to the unis for example!).