I have had the opportunity of being a customer service and sales team leader and I have learned that its all the same thing. The approach is different but tecnically its all the sames and one can transend from one to the other rather easily. For me the most challenging of these two is customer service only because you have to drive your team with little or no incentives and commision unlike sales. That lelment is all about reward and more rewards. In customer service a team leader has to find a way to motivate and drive a team using natural and learned skills and maintain this as well.