It depends on several factors! The first is temperature - if it is chilled out of a fridge then pepsi is better because coke seems more gaseous and sharper from the fridge. At room temp the subtle flavour of coke comes out and its easier to drink, whilst the artificial tastes in Pepsi are amplified and makes it taste a bit worse.
The second is whether you are eating at the time - if you are, then coke wins because it isn't as sweet and is slightly more refreshing, whereas pepsi makes you teeth feel like crap when you take a swig of it after eating.
The third and final is what you drink it out of - out a can, coke rules supreme. Out of a bottle or poured into a glass, pepsi tastes nicer.
So, the ultimate situations are - chilled pepsi from a glass on its own for refreshment, or room temp coke from a can with a fish supper in your other hand. Basically ... I'm no further forward than I started.
I will say that draught coke beats all, not the crap you get to take the edge off vodka in a club, but the nice semi flat stuff you get at the bars in restaurants. That stuff is pure grade A drug.