...I would try to stay away from metal gears. I use to use 9204s and 9402s in the older xcells which needed hi torque just to move the cyclics and pitch. They would start having slop in them just from bench testing and breakin with them installed. After a couple of cases of fuel would need changed. They can also give radio interferance from metal to metal gears.
When you think about how we buy(make or model) and build a heli, where looking for smooth and slopfree mechanics. I also think that for the price of nylon gears, they are cheep to replace in a crash and will probably save the servo itself and other parts by being the weaker link.