I've got both of them and finally had a moment to go and take a set of photo's. Hope they answer the questions your looking for.
As has been pointed out, they use the same chassis, steam chest, pilot and tender. It also appears that the smokebox door is the same. From a size standpoint, while they are different integrated castings, the cabs are basically the same size. So the real difference is the diameter of the various courses of the boiler.
David