| I don't think any one else mentioned it but don't forget about the caffeine in a lot of the diet sodas.
In some people caffeine has a mild diuretic effect and as a result won't get the same hydration effect as one would from plain water. The studies on how caffeine effects glucose have been mixed but many recent studies seem to indicate it impairs glucose metabolism.
Some people also have problems with the acid and phosphorus content of some sodas.
That being said ... I'll put my vote in for diet pepsi, caffeine free of course.
Jason | 
