What to drink when running, is different for each person and definitely has to do with a little personal preference, but there is science behind what you should be drinking before, during and after your run.
Before a run you need to be hydrated, this can usually not be done minutes before a run or a race. Hydration takes time and you should start hydrating at least the night before a run or a race. It is important to get some fluids in your body right before a run or a workout, and I woudl suggest water if your stomach is sensitive. If you have a sensitive stomach and you drink a sports drink like Gatorade right before a run you may upset it. Water is the best bet because it will hydrate and will not upset your stomach at the start of a run.
What to drink while running depends on a number of factors such as distance, or time running for along with weather. In short runs under an hour hydration is not necessary unless running in heat conditions. On shorter runs like these water is sufficient enough. It is when you start running into longer runs of 4+ hours where you need to worry about over hydration, although over hydration can occur on short runs but its not as likely. Over hydration s when you drink to much water and you dilute the sodium in your body. This is when sport drinks such as Gatorade and power aide, come in with electrolytes. If you keep hydrating on a long run and you do not replenish the sodium or electrolytes in your body, your body will start to fail. Sports drinks also have carbs which will need to be consumed on long runs to give you the energy to finish. Another drink to consider on longer runs is acceleraide. Its the only sports drink with protein in it. Drinking this on long runs will help you recover faster, and help you stay hydrated in the process.
If you do not have access to some sports drink to re-fuel your electrolytes on a run you can also replenish electrolytes by eating foods such as pretzels along with drinking just water. Pretzels will also give you carbs as energy to. In some ultra marathon races over 24 hours long they have potatoes you soak in water and roll in salt to help replace electrolytes.
When finished running no matter how long or short you always need to re-hydrate to make sure you are hydrated for the next day. You have to replace all the fluids you lost through sweating. Drinking a sports drink apposed to water after a workout will help replace calorie you burned while running. Acceleraid the sports drink with protein will aid in rebuilding your muscles quicker after a workout. Studies show that protein consumption withing 20 minutes of a workout helps your body recover quicker.
While there is some science behind drinking and running, everyones body is different, and you should do what works best for you.