Winch or regular servo
A Sail Winch Servo is not ideal for driving something that needs to rotate merely 90 degrees: The winch servo is designed to provide a fixed number of rotations from maximum to minimum "angle" setting. More on that later. 

For identifying a suitable servo motor:


Define torque specifications, to help narrow down the search
Search for servos specifying all metal gears: For instance, this one boasts over 15 KG/cm torque.
Give preference to Brushless DC servo motors: They are generally more efficient, generate less EMI, and often are less acoustically noisy as well.
Servo motors specified with metal bearings, or better yet dual metal bearings, can handle more stress outside of the plane of rotation, and they generally will last longer under load. This may or may not be a concern depending on the robotic arm design.
If the torque requirements are not met by hobby servos, look at industrial servo motors, both DC and AC, which can be found for a range of power / torque ratings.
An alternative is to use a stepper motor, if necessary with a gear reduction, if the simplicity of servo control can be dispensed with.


An alternative is mechanical torque multiplication using either a stepper or a conventional servo motor, such as by internal planetary gears:

 (Source and additional information)



A sail-winch servo takes the same servo control pulse duration modulation signals as conventional hobby servos, but is designed such that a "full-scale" traversal is several complete rotations. 

For instance, comparing a +/- 90 degree (i.e. 180 degree) servo with a 3.5 revolution sail winch servo, a signal of ~ 1250 millisecond pulses, i.e. 50% anticlockwise rotation, would achieve around -45 degrees on a regular servo, and around 315 degrees rotation on the sail winch, just short of full circle.

If the control signal were a 2000 millisecond pulse, typically "clockwise maximum", the regular servo would rotate to +90, while the sail winch would rotate through 1.75 revolutions, nearly two full circles.

With this proportionate increase in angular rotation comes the corresponding decrease in precision: If the hypothetical regular servo above could be precisely controlled in 1 degree increments, the sail winch servo's precision would be merely 7 degree increments, by proportion.



The "writing 100 vs 180 simply controls the speed" situation applies to continuous-rotation servos, not sail winch servos. A CR Servo essentially has had its rotation sensor (usually a potentiometer) disconnected, so the controller inside starts working as a pure direction and speed control.