recipes will taste and often look very different if you use one versus the other--richer and more flavorful with the dark. If you don't like that taste or you simple want a lighter taste, use the light.
it should not matter but you could make you own light brown sugar if you have molasses, 1 teaspoon to one cup white sugar as opposed to 2 you just add the molasses to the sugar and stir till it looks like brown sugar it may be bigger crystals though