hsukaria,

It's a good question, what which control surface needs digital servo, but the question arises that every plane has the same control surface but different throws. Apparently I think that 3D flyers need their plane's control surfaces bigger than regular flyers, more throws on their elevator, rudder and aileron. I think it all depends upon your flying and your model which one you are flying. I'm very least experienced here, and I always share my thoughts, I most of the times try to learn from people's experiences as there is no short-cut for experience. Your question is good and I think we all know the answers according to our needs, planes and flying.

Mody