If the metal gears are used on a gyro they will wear out fast because the gyro serrvo moves a lot. I heard on RCU before that the brushes would die out because of all the movement.

If you can get 9402 for cheap then it is okay to use them. If you buy them new then the cost isn't that much different then the 9402's.

The main use of the 9402 is for a steering servo for a car. Mainly 1/8 scale buggies. I used them for that, but am not that competative and a cheap servo was good enough for me.