This thread might get moved to the radio forum, little about aerodynamics.......

The most expensive servos I run are high speed standard digital servos. The largest plane I fly now is powered by a Saito 120. I know the guys that fly the real big stuff might chime in. After holding out for a number of years, this will be my first year flying IMAC.

Say that you want a plane that reacts quickly and predictably, which is a must if you are into any competition. If you take a servo with half the torque (yet same speed) and use half the output arm to apply the same force on the linkage, it will make the control surface move about half as fast to get the same amount of deflection. Sure it still has the same authority to move, but it is much much slower. Torque and force is not the only factor when considering servo set-ups, for many people speed is a huge factor. Also the radios allow the user to program different rates and exponential in the throws. At low rates, the servo might not even come close to it's full potential. But say that it's time for some wild maneuvers and the high rate switch is flipped or the exponential is taken advantage of, then the servos will use more of it's potential.

The newer digitals have more holding power, resolution, and will help larger planes hold trim. Holding trim well is also another characteristic you want if you are going to fly competitively.

I myself like reaction times for the servos around 0.15 seconds to rotate 60 degrees. I love precision flying and a tight feel on the controls. When I hit the ailerons for a point roll, I don't want the plane just to mush around. I want to roll the plane 45 degrees and then stop then roll then stop.....

I have been present when larger aerobatic planes are test flown. I remember one case in which a plane would not hold trim and just would not respond like it should. The plane would wander around and feel mushy with the controls. The owner replaced the servos with high torque digitals and the plane was then responsive and held trim.

Now if you like to lumber around in the skies, you don't need much when it comes to servos. I guess it all depends on your flying style how much you will invest in servos. Also, why not get a little more than you need as an insurance policy, rather than probably barely enough. Buying servos twice for an airplane is pretty expensive.

As an example my small car has 190 HP. I rarely use it all, but when I want to accelerate quickly or drive over a mountain pass (which we have plenty of here) it is nice to have all of that power. Now imagine that my car had half of that power. The transmission could be geared to give me the same torque at the rear wheels, but my quick acceleration and climbing abilities would be a thing of the past. The reason being for that in basic terms is power is equal to torque multiplied by rotating speed. The more power the faster I can turn the wheels with a given torque.

My next car will hopefully have 400 HP. How often will I be able use all of that HP without getting into trouble with the law.....

So basically buy a servo that fits your needs.