I think water is better, but a lot of people won't drink much water. It's better to drink a clear soda than drink too little.
Apparently, they suggest drinking 7up to add sugar to your system during an illness. I would assume as a quick fix for low blood sugar since some people tend not to eat very much if any when they are ill. However, the natural sugar from a piece of fruit would have the same effect and be much healthier. I know that drinking Ginger Ale is recommended because ginger is said to soothe an upset stomach.
7-up is reccomend because the carbonation can sooth an upset stomach... anything crabonated is good for this... but anything with caffine will dehydrate you.
My younger brother when he was little used to call Gingerale "Diarrhea Pop",cause the only time he drank it was when he was feeling ill.