Potassium is an element, so I don't understand what you mean by 'synthetic' or 'natural' potassium... do you have more information on this? Although I'm with you on picking real foods rather than processed things that attempt to mimic what we need...
I'm guessing that's a reference to different molecular forms that contain potassium. Potassium is of course an element, but unless I completely misremember my chemistry, it's not going to be present in a food or drink in atomic or ionic form. Instead it will be present in a molecular form, i.e., potassium chloride, which is what is used in oral rehydration salts. Maybe there's a different naturally occurring form that is better somehow?