Diet soda may be "diet," but it's not any more healthy than regular soda. Soda in general is bad for you, but diet soda has lots of chemicals and GMO's (Genetically Modified Organisms) than regular soda.
I really don't think diet coke is that bad for you. If you are trying to be healthy just go with diet coke. I mean they say aspartame is bad, but they don't know for sure. If you don't care about the calories or sugar go with coke to be safe but if you want to be healthy, go with diet.