I stopped drinking any sodas the day I started lifting (I drank 2l of soda a day before that and was fat-30% bf). Over time they started to disgust me. That's the main reason I don't drink them.
About diet drinks... If you look at calories, is OK. But the main problem is in artificial sweeteners.
I've also read they cause kidney damage and diabetes in studies.
Especially aspartame. It literally destroys your entire organism. My friend always gets headache after drinking diet coke. Aspartame is feces from bacteria Escherichia coli. Enjoy.