As far as I understand, durability = fatigue resistance? I guess that stiffness influences the fatigue resistance a bit. At any rate, tubes rarely fatigue (except perhaps at the butt transition), the joint does, in the heat affected zone. So the brazer's skill is the factor I'd say. But also, I think modern high-strength alloys are a step above cromo or SL, with better grain in the HAZ, and thus better fatigue resistance.
As to cromor vs. SL, they are both pretty much 4130 steel. SL might have a bit more cold work and thus higher strength?