Hum, it's a tough choice. In the long run going with Cycles is better because it leads to a better mastery of Blender, but the learning curve is steep, and the skills involved are vastly different. Since using the node editor is a necessity for better renders for Cycles, you can hold off on introducing Cycles until you start talking about the Node Editor. I would introduce the Node Editor when you start talking about advanced texturing and composite editing. That would be a natural transition and lead in into Cycles. I would not forgo teaching Cycles because skill needed for Cycles are necessary for further development. I hope that helps.