I would say that 15 is a harder replay than 13 tbh. 15 has so much busywork and travel time within it's Big Empty Open World™ that it's frustrating to just want to get to the 5 or 6 moments you actually remember. 13 having a 15 hour tutorial until you hit Gran Pulse is annoying but not a deal breaker since the combat is actually really fun by the end.
