The line comming into your water heater is usually the main line for the house. The water heater uses energy to heat the water and you want to drink cold water; hooking into the hot water would be a double waste of energy. The two lines are only 8" apart so why not go for the cold one.
Now, I know someone will come up with that silly thing about "hot water gets cold quicker than cool water". I have your answer on that.