The good news is that artificial sweeteners (like aspartame and sucralose) found in "diet" or "light" soft drinks won't give you cancer. But large-scale studies indicate that they can lead to weight gain. Increasing evidence suggests that fake sugars activate the brain's reward pathways differently than natural sugars do. As a result, you don't feel as satisfied—and your appetite actually increases.
Switch to juice—or better yet, develop a water habit. If you must drink soda, have a regular and dilute it with seltzer.