This subject gets some play at our field/club. We have one group that says metal gears are a no-no in airplanes and are for cars (and further have the risk of rado interference). Another group that uses them with great success.

I just listen because I've never tried them, preferring to stick with the less expensive nylon/plastic gears. ;-)

I like the S9202 and the S9001 coreless for slightly beefier servos, but the S3001 is mighty hard to beat and does all my throttles regardless of size. The HiTec HS-635HB is a great rudder servo for .40 to .60 size models. I'm usually a Futaba user but I have a couple of those and am very happy with them.