Diet soda is bad news because it contains artificial sweetener, which some people purport has long-term negative consequences such as rotting your tooth enamel and correlating with increased risk for cancer.
Regular soda is bad news because it contains about 150 calories per can, and because it contains buckets of sugar.
For me personally, I would prefer the bad news that comes from regular soda, except... I'm not very good at moderating with sugar. Somehow one can leads to another can, with a side snack of cookies and maybe some ice cream and/or a candy bar later in the afternoon. (It doesn't help that my office has plenty of snacks around for employee morale.) Next thing I know I've eaten about 1,000 calories worth of crap!
I don't do that with diet soda - I can drink a diet soda and be done with my craving. So, even though diet soda might be unhealthy, it's my better alternative between the two.
I can hear you thinking "What about water? That's always a choice." Yes, I drink a lot of water - about 96 oz of water a day. But sometimes that just doesn't do the trick, and I have to scratch a craving's itch...
Your mileage may vary. But as long as you're considering the consequences of each alternative and making choices accordingly, you'll have a better chance of walking the path towards healthy.
- Chris Butterworth