Hmmm... I think I'll have to go with coke XP *dodges tomatoes and empty Pepsi bottles*
To be perfectly honest, I like both, and since I'm far too cheap to buy coke OR pepsi, I go with Waitrose own brand coke XP Tastes just fine, and the only reason I've said Coke is because I am addicted to Dr Pepper, so Coke gets my vote.
To me coke tastes better and has more taste, pepsi tastes like Coke after you've left the lid off for about 2 hours i.e. not totally flat but just lacking any "umph". But then again i'm a cheapskate in the way of drinks (I keep money for icecream and chocolate) so I drink water and cordial juice cos they are way way cheaper ^^