I used to prefer Burger King for breakfast, and McDonald's for everything else... Burger King's hamburgers looked like they had fish eyes in them (no idea why)... I liked McDonald's breakfasts pretty good, but loved Burger King's. XD
Now I can't stand either of them. Going the way of smaller and/or cheaper products, like many others, they're crap now. McDonald's breakfasts are still kind of good, though certainly not like they used to be. =p
I have to go with neither, mainly because I'm not one with fast food (way too greasy and tastes like rubber sometimes). If there's one decent fast food place, it might be Taco Bell (although it's calorie city :X ).