To Master's Spellblade or not to

To all the high level dual wielder out there, how do you determine whether to use it or not?

I always get all the -n% I can get but n reduced is usually a fight between manticore (25) and the spellblade, got a nice plaguebearer one with 45 from a friend, but the question is not as simple as equip the other weapon and compare with master spellblade setup dps times 1.2 (45-25), since the returns for high resistance mobs are way greater than that.

Guess the mobs resistances would probably decide this, especially bosses since trash dies to fast anyways. But we don’t know those and even then experience from the several k hour guys probably beats some computations anyways.

So thoughts?