I presume that it's a 56-powered plane? What size servos did you use? Are they standard size? I've seen some planes of your size call for mini servos, and that scares me. If you have standard sized servos (40-80 oz/in of torque and a hefty servo arm), the gears should not have stripped, even nylon gears. This presumes that your hinging and linkages are good. If you had flutter, the flutter probably caused the gears to strip, not the other way around. I'd make sure that you had a good no-slop setup with your hinges and linkages first.

Of course, metal gears are better (and a bit more expensive). But, if you have a flutter issue, even a metal-gear servo won't help.

Just my humble opinion,

Bob