Go with the fibre gears unless you have double valve springs and will be thrashing the engine abusively.
However, a major source of failure in replacement fibre gears is using the crank gear to hold the fibre one while tightening the nut. You must build a tool to engage the holes on the fibre gear face and hold it that way. I used a piece of 1 1/2 inch by 3/16 angle iron, and welded two stubs of 3/4 round stock on one end, perpendicular to the angle iron, on the same spacing as two of the gear holes. The angle iron is slightly longer than my torque wrench, so I can hold both at the same time. I have used it for twenty years.