The artificial sweetners problem is that it doesn't enter into the body's equation for dealing with sugar properly. It jumps into the middle of the process and sort of short-circuits the brain/body's ability to do the right thing with the calories. So instead of MAYBE using those calories, it stores them. And you train your brain/body to do that over time.
At least with a natural sweetner, even the new ones (and always in moderation, its not permission to go crazy), your body sees it as a food rather than as a chemical.