According to most studies that I've read about, unless you're a top notch athlete, like a marathon runner, or a triathlete, plain water is a great choice to keep most weekend athletes properly hydrated. However, studies did indicate that because of the flavor in sports drinks, athletes are more likely to drink a sports drink, and stay properly hydrated, than they are to drink just plain water and stay properly hydrated.
Now if you're a world class marathon runner, then the carbohydrates, and electrolytes, in a sports drink will improve your performance.