I like being able to call in another ISU-152 if the first one died, although the IS-3 is just better. Another thing I like about the ISU is it's barrage ability. Since the soviets can only get ONE arty firebase, you need all the arty support you can get and unlike katyushas, the ISU's barrage can actually hit stuff! Not to mention it operates better than katyushas outside it's barrage ability as a great killer of tanks and infantry.
I prefer ISU 152, but that's mostly because I like Assault Guns/Tank Hunters more than main battle tanks. However, in urban areas the IS 3 is superior to the ISU because it's more mobile and has a turret, so it's more maneuverable.