Yet they still have caffeine, which isn't exactly "better". Caffeine has caused me to develop insomnia, which is horrible when you have exams and such the next morning and only get three hours of sleep. From what I've read, drinking caffeinated beverages with meals in known to inhibit the absorption of iron and calcium from food, so, that isn't "better" either. I know people try to get out there that diet is better, and sure, it does have less calories, however diet soda has been shown that it may lead to weight gain. When you eat a low-calorie food that tastes like it has a lot of calories, your body ends up craving what it hasn't received, so your appetite may end up stimulated, and cause you to eat more. Drinking sweet beverages, such as soda, may also make you crave other sweet foods, which can also lead to even higher numbers on the scale.
So, again, diet is not "better". Soda is soda, whether diet, or regular, which is why I'm giving it up completely.