What to do with new 4 star effie

I just summoned on the dauntless crimeans banner and summoned myself a 4 star Marth and a 4 star Effie. The effie has +atk -def which i think is great. Problem is, i already have a maxed out effie at 5.40 and am running low on feathers. The maxed effie has +hp and -def. What should i do with my new effie? Should i promote her and merge my old effie into her or wait for black knight?