Compare them side by side. The front flex disk has much less torsional stiffness because it is designed to decouple the driveshaft from the transmission to isolate torsional vibration. As a result, the front disk flexes much more than the rear disk and this eventually causes the rubber to crank. The rear flex disk should outlast the front by at least two to one - maybe as high as four or five to one.

I just consider the front flex disk to be a 60K mile maintenance replacement item.

Duke