Assuming similar specs, the torque is the same, how fast it's available is what's different. The motor position is updated much more frequently from the servo amplifier. Updating the motor position more frequently as Ken correctly pointed out, uses more power. The more frequent motor position updates are what accounts for the increased holding power as well.

I'll add that there are some very fine analog servos and some very ........uh........crappy..........digital servos so that alone shouldn't be your sole decision maker.