Get the whole wheel. This allows swapping the wheel with the hub dynamo for the original wheel for times you will not be needing light. Do you really need a hub dynamo? Bottle and BB type dynamos are much better than those of just a few years ago, drag is much less so slippage is not as bad as it was, Theres no drag at all when the dynamo is disengaged from the tire. If your an all year commuter and deal with snow a hub dynamo is the best option but if you just need light a couple of hours a night a tire driven type maybe all you really need. This would save you having your wheel rebuilt with a hub dynamo or the purchase of a second wheel built around a hub dynamo.