Well, either way you have one GPU that is always working, if the IGP works it gets hot too, but it is cooled by the case fans whereas if the discrete GPU works it's cooled by both the case fans and the HSF of the GPU. Which actually makes the whole rig a little cooler
See, basically, they say that the IGP switches over to the discrete GPU under load but from experience we all know that both GPUs have to stay on right from when the rig boots or one of them won't get active. It's like trying to shift from the IGP in Windows and plugging in a PCIe GPU and expect it to automatically take over the load without a boot.
It's just that one is in a state of sleep but is on.... not off all the way.