It depends which model/car you are intending to install the servo you are going to purchase.

17g Plastic Gear & 17g Metal Gear servo 

- for C14, C24, C54, B14, B24, B16, B36, D12, D42

25g Plastic Gear & 25g Metal Gear servo

- for C34 and C44 only

9g servo

- for 2 speed gearbox only