First of all, Hemp vs. Marijuana is not a competition. Hemp and Marijuana are both in the Cannabis family, but hemp seed does not contain the psychoactive compound, Tetrahydrocannabinol (THC), that Marijuana does.
Hemp is grown for industrial purposes: food, fiber and fuel. This is why hemp is legal and marijuana is not. It is similar to the difference in poppies that are grown to make opium narcotic and the poppies that are grown for the poppy seeds that are used in food products such as bagels.