Rainbow Beans are more effective. One Rainbow Bean has the potential to grow a Pokemon's Affection by three hearts (thus, three Rainbow Beans can completely max out a Pokemon's Affection). Since Docile Pokemon have no preference when it comes to regular Malasadas, it will only gain significant affection from Mythical Malasadas, which are quite difficult to find. So it's probably more efficient to give your Cleffa Rainbow Beans.
Hope I helped. :)
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