I can marginally tell the difference between Coke and Pepsi, but aside from Vanilla Coke, I don't have a preference of one over the other. Diet Pepsi and Coke taste basically the same though, and I seem to drink more of that than the regular stuff.
Any other type of carbonated drink, though, I can tell you the brand almost right away. Whether it be Root Beer, Citrusy (7-Up, Sprite, etc.) or something fruity (Crush, Fanta, C Plus, etc.), I seem to be able to recognize those flavors better.