First, I would do research into any studies about artificial sweeteners and how people may eat more as a result. It would not be a good idea to write about any controversy about diseases and the like, because you would have to address the diseases caused by table sugar, and other than the corollary link between table sugar and diabetes and obesity, the validity of the argument would become watered down.
Also, I think the argument, "Artificial sweetener being worse than table sugar," needs to be modified. There needs to be a specific "for" so that you are indeed comparing something that can be validated. For example: "Artificial sweetener is worse for weight control than table sugar because it leads to weight gain." With this statement, you can show that there is evidence against artificial sweetener use in dieting, if you have that evidence. If you write only, "Artificial sweetener being worse than..." then there is no real basis to compare.
But with your argument as it stands, a counter argument to your argument that artificial sweetener is worse generally than sugar would involve a paragraph that counters this claims and says what proponents of artificial sweetener says in it's favor. A hypothetical (made-up pretend example) counter argument will start something like this (the beginning sentence will be followed by a full paragraph of facts from the supporters of the opposing side of your argument, the sweetener side):
- On the other hand, those who are in favor of artifical sweetener say it is better than sugar because sugar is known to increase weight gain and precipitate hyperactivity in children.