To add a little more to the post on stripped spur gear:
DEFINATELY make sure your clutch is slipping slightly (I think the Nitro
Rustler has one. If not, if it's an option, get it!)
The best way to properly set the gap is to place a strip of writing tablet
paper between the pinion and spur gear, Tighten down the mesh between them
with the paper in there, then remove the paper. 99% of the time this sets
a good gap between the two.
Watch the heavy-duty jumps. This is really *** the spur gear (as well
as the rest of the transmission) until you get your slippage set right.
As far as a metal spur gear - plastic actually gives you a little
insurance against costlier damage. If you're taking out your spur gear,
you're racking up some pretty wild pressure on your drive train. If the
spur gear was metal, that pressure may take itself out on something IN the
tran$mi$$ion (see the $$?) A spur gear, while a pain, is cheaper to fix
than the rest!