I've used the rubber and the cork, the latter for the last six or seven years . No leaks.
The 'cork' is really just evenly shredded cork bits bonded in a rubber compound, not much different than the solid rubber version.
Install either type with a light wipe of silicone sealer on the valve cover side of the gasket. No sealer on the engine side and you can easily remove and replace the rocker cover without damaging the gasket. The gasket stays bonded to the rocker cover.
Tighten evenly, not too much, there are eight bolts at work. That's more than most engines. Often a leak is from a cracked flange. Hard to see, gets worse as you keep tightening. The earlier aluminum covers had much thinner flanges than the later ones.
A regular retorqing of all your various engine fasteners helps to stay ahead of the leaks. Every spring my carb bowl screws will take around a quarter turn to re-snug them.