McDonald's has the better breakfast, fries, and Coffee. For sandwiches, I like the BigMac because of the dressing and sandwich configuration.....but you can't go wrong with having lettuce tomato ketchup and a char-grilled burger. For the burgers, it really depends on which is closer and what kind of mood I'm in. I don't eat either burger often....maybe BK twice a year....McD's 6-8 times/year. I eat breakfast at McDonald's probably twice a month, but mainly because I'm there to get coffee and grab a biscuit for the heck of it(convenience).
EDIT: In reading the responses, I reminded myself of travelling outside of my geographical area. I feel like McD's tends to have better corporate control....meaning, despite the large number of franchises, they take franchising more seriously. I've been to some sketchy BK's in my Interstate travels. I feel like they tend to be a lot more unpredictable. My reviews stay the same...but I'm going to say that my BK and McD's aren't that bad... We used to have 4 BK's and are down to just one in my town and a second one another 5 miles down the road. We have probably 8-10 McD's around and all have been renovated in the past 3-4 years. That level of market saturation and investment should tell you that either McD's is shooting for convenience because at this point, people aren't willing to drive too far out of their way to get to one... For BK, people are going there for variety and because the burgers are good. The one that's left is slammed at lunch....every day.