It's a very good question. A lot of people think they're buying the same thing here but they aren't, at all. KO is more so the marketing and IP (Intellectual Property) company. They sell the syrup. Not a ton of assets, high margins.
COKE on the other hand is the bottler, the distributor etc. It has around half the operating margins and is generally more so the asset heavy business. Lots of equipment, facilities, etc.
The one thing for a company like KO, they don't have much room for improvement. Whereas COKE on the other hand has a ton of ways to improve the margin profile and drive stronger growth. This is exactly what they've done over the years and is one of the main reasons they've crushed KO.
If I were to buy one today, it would be COKE.