MarkT:
Aluminium isn't much / any better than nylon I suspect. Yes you'll pay more
for good hardened steel gearing, but then you'll get what you pay for in power
handling and lifetime.
The sections are thicker though, i have noticed that it is common to pair a larger ally gear with a brass or steel pinion though.
Best power gear i found was a phosphor bronze worm drive of an old wiper motor.
I think they are alll plastic now though.