I'm not an expert but read many studies that your body doesn't know how to break down the sugar substitute like it does actual sugar. Meaning outside of the calories diet soda is WORSE for you than regular soda. Neither is good for you but I highly prefer the taste of regular soda.
Only drink diet when I have to like a couple months before summer starts when I'm trying to shed the pounds I tired to put on over the winter. Bulk/cut type stuff