I don't think that there is a on-size-fits-all answer, necessarily. For example, I'm diabetic. As a diabetic, there is probably nothing worse for me than the high fructose corn syrup that is found in regular Coke (at least the formula that is used in North America. They stopped using real sugar decades ago to save money). Diet Coke has little to no effect on my blood glucose levels.
The long-term effects of the artificial sweetener in Diet Coke is not very well researched, and the research that has been done is fairly controversial. I don't reallt care for the taste of it, honestly and try to avoid it. Sodas made with Splenda seem to taste much better to me.
On the other hand, the dangers of long-term use of high fructose corn syrup are very well known. Among its worst side-effects is obesity, which I'm sure has killed a lot more Americans than artificial sweeteners have.
As a Type 2 diabetic, I can have only have a limited amount of sugar or sweeteners in my diet. I try to avoid high fructose corn syrup entirely. If I do drink coke, I drink the kind that is bottled in Mexico and exported to the United States in glass bottles. In Mexico, Coca-Cola still uses real sugar in their formula, which is better for diabetics (or anyone, really) than high fructose corn syrup.