Well they both make you fat, overweight then obese and you end up getting some heart disease and having a heart attack later in life and dying, and even cancers, (of course this only applies if you eat it too often) and guess what folks? The people in those fast food stores making your "nice" meals don't give a fuck about your health, to them its all about the money and seriously if you did not know this before, I suggest you get out from under whatever rock you have been living under for the past decade.
I rarely eat fast food, but when I do, I don't mind KFC it actually tastes nice, Mcdonalds is feral.