There are 10 main criteria to consider when selecting a servo:
The servo deadband is measured in microseconds, and is an indicator of the "electrical slop" of the servo's electronics. This term is rather technical, but the simplified explanation is that it measures the resolution of the servo.
For example, the standard R/C servo control pulse width varies from 1 millisecond to 2 milliseconds wide, so there is about 1 millisecond of electrical resolution.
The Hitec HS-81 analog servo has a deadband of 8 microseconds. So, the resolution of this servo is about 180 degrees * 8 microseconds / 1000 microseconds or about 0.72 degrees.
The Hitec HS-5245 digital servo has a factory default deadband of about 2.9 microseconds. So, the resolution of this servo is about 90 degrees * 2.9 microseconds / 1000 microseconds or about 0.261 degrees.
Digital servos will have a much higher current draw than analog servos, especially when used for tail control with a heading hold gyro. You may need to use an external switching BEC to avoid overloading the built-in BEC of the ESC, which will add to the weight of the heli.
Also, some servos which have high torque will have a correspondingly high current draw. For example, the GWS Naro HP BB has the nickname of "current hog" because it draws a large amount of current for its size.
There are four servo sizes which are tyically used on electric helis: Standard, mini, micro, and submicro.
The standard servos are usually 40mm x 20mm x 40mm, and are typically used on helis which heavier than 2.5 kgs, such as the Swift and Logo 20.
Mini servos are about 30mm x 12mm x 30mm, such as the Hitec HS-81 and HS-85. These servos are typically used on helis between 1kg and 2.5kgs, such as the Viper 90, ECO 8, Logo 10, and Robbe Spirit.
Micro servos are about 23mm x 12mm x 24mm, such as the Hitec HS-56. These servos are typically used on helis between 500 and 800 grams, such as the Zoom 400, T-rex 450XL, and Viper 70.
Submicros vary in size, but the Hitec HS-50 and HS-55 are in this category. These servos are typically used for helis under 400 grams, such as the Piccolo, GWS Dragonfly, Century Hummingbird, etc.
Some helis which use micro servos have four holes for mounting, and some have two.
Some servos are very heavy. This weight may be undesirable for a heli which is optimized for duration flight.
Speed and torque are dependent on the gearing inside the servo. Often there will be two servos from the same manufacturer which be visually identical but have different speed and torque specifications, such as the HS-85 and Hs-81. This difference is due to the difference in gear ratios inside the servo.
Torque is usually more important for swashplate servos because it is important to hold the swashplate position accurately. Speed is more important for tail servos when used with a heading hold gyro which requires fast servo response.
There are two main types of gears used in servos: plastic and metal.
The advantage of plastic gears is they wear slowly. However, they are easily damaged in a crash. Metal gears are more durable and are more resistant to crash damage. However, they wear much faster and develop mechanical slop.
Therefore, it is best to use metal geared servos for swashplate control, where the crash resistance is desirable. The plastic geared servos work better for tail control because tail control slop will cause tail wag with a heading hold gyro.
Servo gears are sometimes destroyed in heli crashes, so you should consider the availability of replacement servo gears when selecting a servo.
Slop is undesirable for both swashplate and tail control servos. SOme servos, such as the HS-50, have excessive amounts of mechanical slop, which make them undesirable for helicopter use.
The two best servo manufacturers are probably Volz and Multiplex. They have a history of manufacturing servos with extremely good geartrains and very tight deadbands.
The next best servo manufacturers are probably Futaba, JR, and Hitec. All three manufacture good quality servos.