Diet soda can actually help you gain weight in its own way: since it's sweet, but has no calories of its own, it tricks your body into craving the "missing" calories it expects. The effect is an increased appetite coupled with an insulin spike that results in extra fat storage.
However, many people simply don't think it tastes as good as regular soda, which is actually genetically determined, much like the cilantro/soap situation.
Regular sugar is also an insulin trigger itself...plus it has real calories, lol.
And there's been a bit of a scare for decades over whether artificial sweetener is linked to cancer...the short answer is yes, but only if you eat a five-kilogram brick of it every day, according to studies done on lab animals.
The bottom line I'm getting at is, either one will help you gain weight, so just pick whichever you like better is my take.
1 year