Servos for flybarless usage
A rule of thumb to find well suited servos for you helicopter:
As strong as necessary, as fast as possible.
Servo torque in kg/cm should be at least 2-3 times the heli weight (in kilogram). Speed should be at least 0.1s/45° or even faster. In addition the servo should be very precise, have little backlash and a low deadband. And of course best would be if it is a digital servo and supports 200 Hz driving frequency. Also using special heli servos which are designed for maximum vibration resistance is a good choice.
Note that all the speed is useless if the torque is not enough. In doubt when two servos of a series are only slightly different in speed and torque, normaly the more powerful is in advantage.
Anyhow ask the manufacturer of the servo if it can be used in combination with flybarless/gyro systems. Not all servos can handle the additional stress of getting control signals every few milliseconds. Also it must be said that not all servos can be used at maximum frequency even if so it is told on the servo's data sheet. Depending on air temperature, voltage level and heli size it is possible that servos can not be driven at their limit.