Being lactose intolerant I can say for a fact that milk is extremely hard on the digestive system, not only the milk sugars and proteins but yes the calcium as well. Soy is supposed to be better but I really don't know I haven't seen or heard any hard evidence saying its any better or worse. But my doctor suggests using soy products, which I don't I am only mildly lactose intolerant and I love my ice cream so I deal with the repercussions
I've also been told goat milk is better for you than either cow milk or soy, but I'm not about to make that switch without some proof