This is a hard one. No personal preference, so actual battle!
I'm going to give it to Clefable because of Meteor Mash. They both get a steel-move, only one, but Wigglytuff's is Gyro Ball. Both of them are relatively slow and have a close speed stat, so it wouldn't be too powerful. Also, Clefable gets Unaware, so any stat buff/debuffs Wiggly uses are ignored!
Aria Meloetta vs Piroutte Meloetta?