Answer:
Step-by-step explanation:
There could be. Not much of the artificial sweeteners are used in the diet colas. (This is measured in mg, which is gram broken down into thousands).
On the other hand a 355 red can of coke has a about 9 grams of sugar in it. That is a great deal. It may make the can of "natural coke" heavier enough to make a difference in it's buoyancy. It could allow the natural coke to sink a bit below the surface.
It is an ideal test for a science fair project for a student in the middle grades.