It's no-Brainer to switch to plain carbonated water, I wouldn't say it's a no brainer to switch to sugar free, that depends on how much soda you drink and how badly the sugar-free one affects your hunger, to me it's a lot, so if I would want a can of coke I would probably better of sticking to the normal one since I don't drink it often and it won't throw off my hunger to a crazy degree.

Though to be fair you might not have the context of how much diet soda hrows my hunger off, it's so bad for me I can easily eat my 800 calorie high protein chicken-wrap 2 hours after the previous one if I drink a diet soda with it, while without the diet soda I can go for hours without feeling hungry