I don't exactly prefer one over the other, but assuming this situation was in my area I would go to McDonalds for one simple reason:

All the BKs around here are super slow. Keep in mind that I am absurdly patient when it comes to food service. In fact, I will occasionally call out another customer if I feel they are acting like a brat because someone made a slight error or that they didn't get their food in an unrealistic amount of time.

But there is no way ordering a burger with a regular sized fry should take 10 to 20 minutes. Every single time I tried. I figured maybe it was just one particular location. So when my girlfriend and I went to go eat, she felt like Burger King and we went in, but it was at a different location. There were three people working. The manager hid in the back. He said he would be with us in a moment. He continued to hide in the back. We left.

Okay, so maybe it was that area in general that sucked. The one by my parent's house was always cool when I was a kid. NOPE! Inattentive workers and another 20 minute wait for a very simple order.

Haven't been to one since.