Actually on fitness sites diet soda's have been known to cause weight gain for years..these are people who write down everything they eat and drink and when they drink a lot of diet soda their weight goes back up. I know it does for me if I drink 2-3 a day and I keep track of every calorie even my supps I take.
I can't remember the exact science but heavy aspartame exposure directly contributes to increased blood glucose levels which leads to increased fat.
I don't really care for aspartame due to taste. Sucralose AKA Splenda has been determined to be safe and has had over 25 years scrunity. Pepsi used to make a drink called Pepsi One and it was sweetened by Sucralose. Ingles in store brand of diet Cola also uses it and it taste pretty good. Personally I've never had any trouble loosing weight using Splenda. But I'll pack pounds on fast if I go back to regular sugar drinks. Sucralose is made from sugar BTW.