The servo position is controlled by an electrical pulse or variable duration which in electronic terms is called pulse width. The pulse repeats at constant intervals and this repetition frequency is also known as frame rate and is represented in Hertz (Hz). The majority of RC receivers will produce a frame rate of around 50Hz which means that they will instruct the servo where to go 50 times each second. The position of the servo horn is defined by the pulse duration. For a typical RC servo a pulse duration of 1520uSec means that the horn should be at the mid-point of the servo travel. If the pulse duration becomes shorter or longer the servo horn will move clockwise or counter-clockwise respectively.
As gyros evolved it was apparent that 50Hz was not enough to achieve the desired performance as the time intervals between pulses were too long. So manufacturers starting pushing the frame rate up. JR gyros went to 250Hz and Futaba pushed as high as 333Hz. This allowed the gyro to talk to the servo more frequently but the position command remained the same (1520uSec still means mid-point).
Then the GY601/611 gyros came along with the non standard 760uSec system which is also known as "narrow pulse". With this system the servo travel mid-point is at 760uSec as opposed to 1520uSec. In theory by shortening the pulse to half it can reach the servo faster and therefore the servo can react sooner. In reality the benefit is negligible as the savings are less than 1mSec. Considering the inertia of the motor windings and the helicopter as a whole 1mSec does not give any practical benefit and very likely this system was only implemented to tie specific servos to these gyros.
So when buying a new tail servo the important things to look at are speed, torque and capability for high frame rates. With the exception of the older JR servos (8700G etc) most new digital servos will work at 333Hz. This includes all Futaba digital servos and therefore the S9254 will work as good as the S9256 since they have the same torque and speed spec but perhaps a little hotter due to the lack of metal case.
Added (March 2009)
As gyro technology evolves Spartan RC has introduced a new 560Hz servo pulse mode in the V2 firmware update of our gyros. This higher pulse frequency is only possible when using narrow pulse (760uSec) servos such as the S9256 and BLS251. Because the servo position is updated 68% more frequently this system has the potential to deliver smoother tail response and more consistent pirouette rates.