It depends on the table you're playing. Tornado balls roll true and are not as susceptible to nicks and scratches. They're too heavy for some german style tables. You could still use them but the table wouldn't play right. Cork balls are fun, especially when they get a little wear on them, but honestly they're not much good.
First, determine what table you're buying them for. Then try to find the original ball for that table. You could buy a couple of each kind of ball just to experiment. Check out the store on this site.