I think I touched on this bit in the above response. The rule of thumb, I believe, would be This: if you want a more tender cake, use the whole egg foam method. If you want a drier cake, use the separated egg foam method. For example, when I make sponge cake to be dipped in syrup and then used as a dessert component, I use the separated egg foam method: foaming the whites + half the sugar and the yolks + the other half of the sugar separately, and then folding both together with the dry ingredients (usually without additional leavening). This results in a ridiculously dry cake that can be soaked in syrup indefinitely (within reason) and can literally be (gently) squeezed out like a sponge without dissolving into a nasty, pasty mess. (See: these ladyfingers) A genoise, however, which is based on whole egg foam, become very fragile once “syruped.” For this reason, I like to use genoise when the cake is pretty much going to stand alone (other than some icing). I like the separated egg foam method when the cake will be one layer in another dessert, as in a trifle or tiramisu.