Which refinement would work better on my Summer Robin?

I've managed to get an extra legendary Ike which I foddered for his warding breath, giving it Summer Robin, she's -ATK +SPD. However, I'm not sure what refinement to give her. With a RES refinement she reaches 35 RES and 39 RES with Warding Breath, however she'll have 39 base speed. With a SPD refinement, she'll have 42 as her base SPD, and 31 RES.

My question is, which lance would be more effective: Slaying Lance(+RES) or Slaying Lance(+SPD)?