"Lights moved restlessly along the bank of the river..."
Local colour is a literature style that consisted in describing the local enviroment, the peculiarities of certain location. It was very popular after civil war in the United States and from 1860´s till 1900´s was the most common style of literature.
Well it seems the 18x48 is bigger so personally id buy the biggest one but im not sure of how much money your willing to spend on this so if you want the cheaper one go with 12x30