Corrin Yato Refinement

Why not go with the effect and have him support his support unit even more?

Plus it's +2 to each of his stats.

I do believe that the unique upgrade is best in my honest opinion, however if you lakc the Divine Dew and or don't wish to use it on him (you might since he's your summoner support) then he will benefit from it (+2 to each stat, +3 hp and +4 to each stat to his allies)