In my experience rubber is far better at sealing than cork. Cork doesn't seem to seal good enough especially on steel valve covers or sumps where its not unusual to have some level of warping going on due to previous abuse. Also for me at least cork doesn't provide as good a feeling when tightening so if youre not using a torque wrench its easier to overtighten the bolts compared to rubber creating said warpage and leaks. In short given the choice I would always prefer to use a rubber gasket.