Should I get a Teleconverter of a Telephoto Lense for Shooting Sports?

I love taking sports photos, and am looking to make a business out of it. Currently I have a 50-150mm lens and a 18-40mm lens. The 150mm doesn't quite cut it for most sports, because of the distance from the sidelines to the players (ie: baseball, football, auto racing). So I was wondering what would be a better choice: Buying a 2x teleconverter to help my current telephoto lens, or would a better telephoto lens (say a 300mm) be the best choice?