Rubber gasket vs. a cork gasket

I don't remember what I used last time when I put the valve cover on, but when I was trying to get a carburetor to work, the rubber gaskets that come with the rebuild kits and in all of the remanufactured ones I bought wouldn't seal up the gas bowl (Holley 1920). I had to buy cork gaskets from NAPA to get them to stop leaking.