Fiber gears are quieter, but aluminum lasts a little longer. Unless you're racing or planning to run a heavily-loaded truck wide open all day, either one will outlive the average hobbyist. There's not enough difference to matter, as long as you start with a new gear on both the crankshaft and the cam. Don't trust the old fiber gear- - - -replace it with whichever style cranks your tractor.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!