The answer is “it depends” - on how you are using it. For serving alongside your favorite rolls or biscuits, they are both delicious! In sandwiches, you can use either but jam is easier to spread than preserves, since the fruit pieces are crushed in jam while left whole in preserves. For baking, again, it depends on what you are making. Not only can the flavor vary (preserves are typically more “fruity” tasting) but also sometimes the consistency, too. The safest approach is to follow the recipe!