Hi Jace! Great question. It was such a great question, it inspired me to write an entire post rather than just a reply. You can read the whole post here:
If you just want the TAKEAWAY, read below...
Water is best. If you do love soda, regular or diet soda should be consumed as a treat rather than made into a lifestyle staple. If you are trying to lose weight, diet soda could be the better alternative as long as it is used short-term and at for a moderate use. Both regular and diet soda are possibly linked to increased chances of metabolic syndrome which increases your chances of dying from coronary heart disease.
Diet soda may have a link to weight gain because of possible disturbances caused in the brain and gut flora (but more research is needed), but such data is usually revealed with high consumption and diet soda use for extended periods of time.
Regular soda on the other hand is more clear cut because it is sweetened with sugar rather than artificial sweeteners. Let me be clear though, regular soda does not cause weight gain, excess calories (from drinking regular soda) is what can cause an individual to gain weight.