With fresh, you're adding vegetal matter with the risk you could extract some vegetal flavors. Should be relatively low, though. Light or dark sugar is your call. Dark sugar or even brown sugar will add some flavor but on balance very little. Taste the sugars if your store will let you - the bulk foods aisle is great for this - and see what you want in your beer. If all you're doing is cutting body, pure cane sugar will work. If you want burnt sugar and dark raisin flavors, use or make your own invert syrup (candi syrup). The candi syrup "rocks" are slightly impure sucrose and not worth the money.