Threaten Res or Spd?

So my team consists of both original Robins and Corrins, though none of them have incredible speed or extreme damage output, so I want a good Threaten for a C skill. Spd helps all 4 of the units, while Res helps 3 of them. However, in the off chance that the Spd reduction doesn't grant a double, Res helps bolster the damage dealt in a single blow.