John First off, you wouldn't have much luck trying to grow crystals of flour. It's an organic substance that doesn't crystalize.
Sugar is also an organic substance, but it is different than flour and does crystalize.
Salt is inorganic, a mineral, and also crystalizes. Crystal growth rates for both sugar and salt depend on the concentration of the solution you start with and the temperature it is crystalized at. By varying the solution and the temperature you could get either one to grow faster than the other. There's no hard and fast rate. I'm not positive, but I think that if conditions for both are optimal the salt will grow faster.