What Lens do you think is better for Wildlife?

Ok i can get either one so price does not really matter, I want to get the Sigma 50-500mm Lens with a 2x Converter so it will be 100-1000mm if you know of any compatable with it plz let me know by the way or the Canon 100-400mm with the 2x Convert for a 200-800mm so which one do you think is better for Wildlife and Sports Photography? I cant decide because the Sigma has AFO so it will have sharper images but Canon has IS for faster shutter Sigma weighs but it also has further focal length. So just plz tell me what you think. And i am currently using the Canon 40D.