All those "diet" drinks have sugar replacements like aspartame that are dangerous and if taken in larger amounts during longer periods can cause various shit including cancer.
So better drink real coke but limit your self to a smaller amount of it per day and drink more water instead.
You can read more 'bout it at Wiki:
http://en.wikipedia.org/wiki/Diet_co..._health_issues
http://en.wikipedia.org/wiki/Aspartame
http://en.wikipedia.org/wiki/Aspartame_controversy
Here's a short copy paste from Wikipedia:
Aspartame, which has been blamed by some scientists and medical professionals for possibly causing serious illnesses (such as
cancer, brain tumors, brain lesions, and lymphoma) when consumed in large quantities. One of the chemicals produced by aspartame after ingestion is methanol.