The 2123 is also a FET servo
FET is just a certain type of transistors. Field Effect Transistor. They are more efficient than normal transistors. Once upon a time, they were hot news, now they doesnt cost much and are quite common. They are also used in many chargers.
Another difference between the 2123 and the 2113, is that the 2113 use plastic gears and the 2123 use metal gears. Metal gears can take more abuse, and I believe they wont get out of shape as fast as plastic types.
The 2123 got a coreless motor, which the 2113 doesnt. A coreless motor means smoother and faster operation. Usually metal gears slow down the servo, but probably the coreless compensate for that.
Digital versus analog; A digital servo got much better resolution. The torque are equal, regardless of position. And the holding power are huge. However, a digital servo use a little more power than a analog. How much? Well, I havent heard of anyone for whom it was a problem.
Now, the question is: Are the 2123 worth the extra money? As far as I know, when one first have tried a digital servo, they wont go back. Personally I had to wait two months for my 2123 - I've never regret it ! This feeling of control, suddenly I was able to change direction in the middle of a chicane. And to steer contra, when the car slide.
The 2123 are not cheap. And the 2113 aint bad. I will say: Buy the best you can afford. You can take it to the next car. And the next. And....
Here you can read more about KO servos: KO servos
And about FET servos: FET servos