I assume that the spring gives you better efficiency (a motor driven backwards is a generator; but not necessarily a good one, and battery charging doesn’t help); but motors have the advantage of being exquisitely controllable. Unless you are talking some retro brushed job or something, you can control the current flow through each segment of coil as your application requires, moving between near-zero resistance and maximum torque(in either direction) at lightning speed.
Springs have simplicity and efficiency; but if you really want to be pushing off now and the spring in your joint insists that it is going to be soaking up all your effort until it reaches it’s set tension, not much to be done.
Now, a system capable of dynamically modifying the springiness of materials, or of engaging/disengaging elastic components from the limb on demand… That would be quite a toy.
