I think Burger King is the better fast food joint out of the two of these. The burgers taste better, the fries taste better, the have WAY better pies and the only thing McDonalds does better than them is the chicken nuggets.
My preference over both of them is the Unlucky Fried Kitten (KFC). If it's chicken you want neither McD's nor BK come anywhere near close to KFC. KFC fries do tend to be a bit soggy though. Last time I had a McD burger (I used to eat them quite regularly!) I could taste the damn thing for three days after the event. YUK!