When it comes to prepaid cell phone plans, the advantage is that you are incomplete control as to how many minutes your kid uses and they cannot go over them.
In my opinion a prepaid cell phone plan will control how much your 15-year old will spend on calling. If you are concerned about the cost, that is the best way to control it.
The major advantage is that you know EXACTLY what you will be paying. No surprises when that monthly bill comes in that lead to a battle between you and your teen.