Salt water boils slightly faster than sugar water. In actuality, both of these substances raise the boiling point of water, making it take longer for the water to begin to boil, according to Serious Eats.
Because salt and sugar are dissolved solids, they increase the boiling point of water by less than one degree. The effect of salt making water boil rapidly is due to nucleation sites that make it very easy for bubbles to form in a pot and escape. These bubbles form on microscopic scratches on the surface of the pot, giving the volume of water an irregularity that increases when salt is added to the mix.