As long as you stick with Futaba, Hitec or JR, you'll be fine. Shop for a metal gear 100oz/in (or more) with less than .2 sec/60 deg at the voltage you're going to run. One thing to keep in mind is the mechanical set up. Servo torque specs are rated at 1" from the center of the output shaft. Rudder pull/pull set ups can have 3" long arms or so which significantly reduced the torque applied, so keep that in the back of your mind.