RE: whats the difference from regular and digital servos
You can tell the difference in the air, at least I usually can. Digitals center better, although so do the higher priced coreless servos. The main thing is the holding power. As has been mentioned, just a small amount of movement will get you full torque, whereas an analog servo may take up to 20 degrees of movement before full torque comes in for holding a control in place. I like them especially on rudder on fun fly and 3D planes.
If you can tell when you have a worn servo that doesn't center or trim as well as a new one, you'll be able to tell. I do admit that most people have a lot of fun flying the low priced, standard servos and don't worry about a little out of trim condition or the fact that the ailerons don't center the same from different directions.