Pop and diet pop are almost as equally as bad for you. (I say almost, because some regular pop is actually made with real sugar.)

If you've watched documentaries and lectures like Sugar: The Bitter Truth and Fed Up, you've probably come to the conclusion that most people are consuming way too much sugar. When you consume sugar (fake or real) it puts a heavier load on both your liver and pancreas and doesn't make you feel more hungry, it just doesn't tell your body when you're full, which is why it's better just to avoid it all together.

IN the same sense that you SHOULD avoid alcohol together, but let's be real here... Is it okay from time to time? Absolutely, everything in moderation. You're eating out with some friends, fast food water tastes like butthole, so you get a small pop instead. A couple cans every week? Probably won't do much harm. Consuming a 2L every day? Probably a bit excessive.

Here's where the problem really lies... Sugar can become addictive, which is why it's so difficult for people to stop drinking soda pop, even if they start out in small doses and they can fall into this cycle and that's when diet soda comes in. It's not diet at all, if anything, it's worse for you because your body never actually gets REAL sugar. Don't let the diet trick you, it's still terrible for you in the same sense that alcohol is terrible for you.

Sources:

Sugar the Bitter Truth: https://youtu.be/dBnniua6-oM

FedUp (Trailer, full length movie is on Netflix): https://youtu.be/aCUbvOwwfWM

Sugar more addictive than cocaine: http://www.nydailynews.com/life-style/health/white-poison-danger-sugar-beat-article-1.1605232

tldr; It's okay in moderation.