I've always thought of it like this.
I want the Absorb early in the fight (lots of minions) to help eat the alpha and or give me a regen window, depending on the timing. Hence, putting it into one of my 2 PBAoEs. FSC got Armageddon for the Purple proc, so Combustion got SGF.
I want the +Resist in long hard fights vs a small number of the remaining hard hitting enemies. I'm more likely to be using ST attacks and maybe not be using any AoEs at all. Hence, Incinerate got Hecatomb for the purple damage proc, and then I had the choice of SMotT in either Scorch or Fire Sword. I went with Scorch, even though its proc chance is lower, because it literally goes off twice as often. The average procs over time was higher in Scorch due to this.
That may not be the right way to do it, but it was my take on it.