sports drink vs water
I never use a sports drink during training. I've found you become dependent on the extra kick it gives you and eventually actually makes you weaker. I use them during competitive events for the extra kick.
Here's my thinking:
When training, use only water because you become stronger in and of your own bodies ability to adapt. When competing, use a sports drink for an extra kick that helps your body and gives you a psychological boost because you know that you're getting a physical response from the sports drink.
I do however use sports drinks AFTER a long hard training session to aid recovery, but never during.
For the record I compete in mountain bike racing and duathalons, the disclaimer being I have only my own results as proof, but they span sport performance over 15 years of competing.