Cork or rubber?

The rubber are better I believe, from experience.

The cork also lends itself to some very unwelcome and greatly disturbing events, such as the overtightening of your aluminum valve covers (like Mopar perf., for example), and the subsequent cracking of the same when they get nice and hot. Peachy. Real peachy.

You can overtighten them, and have little idea that you've done it. I'll never buy them again.........and they leaked like crud.....just little bits, but really annoying.