RE: When are digital servos justified?
Where high torque and high precision are required.
Helicopter, 3D and very aerobatic planes benefit from the speed and precision.
Trainers and T28's not so much.
As an aside, there are some very good analog servos and some cheap crappy digital servos so the fact that it's analog or digital may not tell you the whole story.
For a DC-3 (assuming we are talking about the prop driven air liner) any servo that meets the torque spec's would be fine, you wouldn't see much difference with digital servos. With a Yak or Funtana or something it would be like night and day