And I would never have believed that I would be advised away from digital servos (being a computer-engineer my self). The mantra has been:"digital is the way to go", "digital is the future!" ...
I've been told that they are superior in every way to non-digital servos when it comes to speed, power and precision. It's just that I have personally handled four different types and I wouldn't have put any of them into one of my models. Two of them interfered with the other servos in the circuit, despite having ferrite-ring suppression coils, and the other two sat and twitched/trembled all of the time, which I found unnerving! That said, they were all from the cheap end of the market and I suppose if I were to pay a premium price then I would get a premium product - but I'm not inclined to shell out thirty or forty quid for something that I don't really need.
This is my opinion only - I don't want to start a war. Suit yourself, Hande!
BTW TF's reference to 'elevation' is to the usual function of the two sticks in a model aircraft. Depending on the stick mode, one will operate the throttle when moved up and down, while the other will operate the elevators. Conventionally the throttle stick will have a ratchet rather than a centering spring, but for use in a model boat (where the motor stops at centre-stick) it's much better to fit a spring to that stick. You will almost certainly find the corresponding receiver outputs labelled with 'Thr' and 'Elev' or similar.
Dave M