Tea and coffee can cause dehydration because caffeine is a diuretic, not because of the tea itself. Decaf would not produce that result, but of course the taste is quite different.
Similarly sugar draws the water out of your cells, so sugared soda really doesn't quench your thirst. Combine that with caffeine, as in Coke, and you have a diuretic. Diet soda without caffeine would not be a diuretic. The other thing that sugar does is give you a temporary "sugar high," then it drops your blood sugar down to the basement. Some people get headaches from too much suga for that reason..
Aspartame is NOT cancerous, and I suggest you read some research before making such statements.
http://www.medicinenet.com/script/main/ ... ekey=82425
Cancer was related many years ago to saccharine, the ingredient in Sweet n Low. But they had to feed the test animals incredible amounts to produce that result, the equivalent of a human ingesting pounds of the stuff per day. That's why it's still on the market.
As far as getting headaches from certain teas, it might be an allergy to a particular ingredient. Rooibos is made from several ingredients, and the combo also varies with vendors. I don't care for the taste, but it's not poisonous to the general population. If you can narrow down the ingredient, you want to avoid it in other herbal teas.