Either will do nicely. The main difference is that wood chips burn faster than wood chunks. If you plan to add just a handful or two of wood chips for a light smoke over 20 minutes or so, the burn rate is not really an issue. But if you would like to smoke your food for an hour or more, it might be more convenient to use wood chunks. Depending on their size and the amount of air getting to the fire, they will burn that long or even longer. If you soak wood chips in water for at least 30 minutes before adding them to the fire (there’s no need to soak wood chunks), you will significantly prolong their burn and they will smoulder more than flame. Just be sure to drain the wood of water first so you that don’t extinguish the fire.