Does it work best to substitute different flours (like Unbleached All purpose for Bleached Cake) by weight or by volume?

I can see reasons both ways. There's the same "mass" if you go by weight.  But the argument the other way is that it is the volume of the flour that adds structure and substance to the cake-- so  a cup of one should use a cup of the other despite the weight difference.  

Notice I'm not asking adding in corn or potato starch for some of the other flour. And, yes, I realize the texture of the cakes.  I'm just wondering about how much to use (and why).

Forgive my detailed answer but it is also for the benefit ( hopefully, haha) of someone not aware of differences between the flours.

I measure by volume (displacement) which you could still figure out by weighing the types of flour that displace space in cups and tablespoons.

So if you have all purpose flour and your recipe calls for cake flour - for every cup of cake flour called for  - use 1 cup less 2 tablepoons of all purpose flour.

You only have cake flour but the recipe calls for all purpose flour - for every cup of cake flour called for - use 1 cup and 2 tablespoons of cake flour.

The real question is why? The answer is that those two tablespoons either way are to make up for the difference in the protein content of the two flours. The protein content variance is because cake flour is made with soft wheat flour - lower protein. All purpose flour is a combination of both soft and hard wheats so the protein content is higher.

All purpose flour has a higher protein content so when you use it instead of cake flour you are removing two tablespoons of flour to reduce the protein (gluten) content.

Cake flour has a lower protein content than all purpose flour so you increase the amount of flour called for when substituting cake flour for all purpose flour.

Cake flour produces a more tender and delicate crumb because it has less protein and therefore creates a more delicate structure.

All purpose flour has more protein and creates a stronger structure.

With regards to substituting unbleached all purpose flour for bleached all purpose flour - you substitute the same amount by weight or displacement but results may vary. That is because the bleaching process affects protein counts.

Substituting unbleached all purpose flour for bleached cake flour - now that is where it gets tricky. Bleached flour - cake and all purpose - has less protein than unbleached flour. The amount to substitute - I don't know and have not seen an equivalent. Also there is now unbleached cake flour which will give a bit more structure than bleached cake flour and that would be tricky too.