For rudder he actually recommends 8411SA (plastic gear) in the manual. For the rest tail surfaces 9411 but there is a reason behind that. The Saito 100 is not a very powerful engine for 3D IMHO, as a result he tries to save some weight by using much lighter servos than the 8411. If someone uses a stronger motor such as the YS-110/120 he could actually go for metal gear servos to be in the safe side. This what I did actually I chose to use the Japanese version of 8411 the 8425 with metal gears at the tail and HiTec metal gears on ailerons.

If you go to the Showtime thread you will read about some rare flutter incidents due to wrong servo selection or poor mechanical advantage. Most of the guys who fly pure 3D did not have problems with metal gears. I have not flown it yet so I can not tell you my experience at this point, but I hope not to have problems with this set up.[8D]

Rgrds