RE: Hitec coreless or hitec digital?
In more simpler terms, a digital servo, regardless of the moter, typically has a smoother control curve because it steps at 1us instead of the standard 8us you get with an analog servo.
This results is more precision for the digital servo and should be the reason you move up to one.
On the other hand, analog servos with coreless motors are inherently more precise and have more torgue than 5-pole or 3-pole motors.
If your looking for precision and high torque but don't need to match servos to the same control surface, you don't need digitals. If matching servos to the same surface is your primary purpose go with the digital servo.
If you want the best of both worlds, maximum precision and maximum torgue, go with the coreless digital.
If you want maximum wear (metal gear slops over time) with your other maximums check out the new Hitec Karbonite-geared digital coreless servo, the HS-5975, at $81.95/ea. Why would anyone choose an alternative at 40% more cost?