Well Considering they have a lot of similarities like they are both normal type Pokemon they both need a Moon Stone to evolve into the final evolution (Clefable and Wigglytuff),and how both their first or "baby" forms were made in the Johto Region (Cleffa and Igglybuff) and how they are both found around mount moon though Clefairy is found in Mount Moon in the Kanto..so I'd honestly consider them to be equal but if you mean in training I'd think that depends on the trainer,how the Pokemon was trained and so on.
Actually, Clefairy is only Fairy type, while Jigglypuff is Normal/Fairy, and Clefairy can have "Magic Bounce", a very useful ability. Therefore, I think the "Clef" family is far more important in combat u-u
