Coca-Cola only. Pepsi's sweetness is much more artificial and while I like it for the first few sips, I have trouble finishing even a small can.
The same reason I hate Dr. Pepper and all the diet versions of both Pepsi and Coke - I can't stand the taste of the artificial sweeteners, it's undrinkable to me. Ufortunately, Sprite is now being sold here with artificial sweeteners only and the taste has gone to shit. Though I admit in case of Sprite I find the taste somewhat more tolerable, so I can actually finish the small bottle.
On the other hand I like Mirinda much more than Fanta.