DO NOT substitute sugar for raisins on your cereal. Not only is adding table sugar to your cereal not a good idea because of the added calories, but it's just not necessary. A single teaspoon a day adds up over the course of a year to more than a pound and a half of added weight as fat (the assumption here is that it's stored as excess calories).
The more interesting part of your question is whether or not raisins on your cereal are a good idea. It's true, raisins make your cereal taste better but it's because they, too, are high in sugar. Although not the same kind of sugar as the processed white stuff, they do bring just as many calories per serving which is exactly why they're added to trail mix and are popular with hikers as a high density energy source.
On the other hand, raisins offer more than just sweetness. Raisins are a good source of fiber and contribute to your total daily servings of fruit. Additionally, as raisins are nothing more than dried grapes, they offer some of the potentially beneficial effects of compounds known as phenolic antioxidants. Other cereal topping options include berries of any kind, diced apple, and sliced banana. If you absolutely need your sugar fix in the morning, give agave nectar a try, it's a low glycemic, natural sweetener.