Many people think that Digitals are stronger and faster than analogue, but speed and power as mentioned is dependant upon the motor and gearing used in the servo (just like any servo).

The increase is due to the way that digitals work. In simple terms, both digis and analogue use a feedback loop to get the servo arm to move to the desired position. The difference is that with digis it is either on or off, so 100% of the force is applied as soon as a difference is noticed, while with an analogue, it builds up as the difference increases.

This gives the digi the holding power and centering that it is known for. It is what also causes the hum that digis are also known for, the weight of the control surface will cause the servo arm to deflect ever so slightly, the servo picks this up deflection, and corrects it with 100% of power.

In an analogue, it will find a happy medium where the force of the deflected contol surface counteracts the correcting force being applied by the servo.

Personally I do think that digis will become far more widespread over the next couple of years, but will not replace the analog servos completely for some years after that.