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.