How far are you going to have to go in Tokyo if you're craving Mexican? And is there even a decent Mexican restaurant in Tokyo? (I'm guessing the answer might be yes but not many.)
On the other hand, I understand KFC is very popular in Japan which happens to be one of my favorite US fast foods if I really need to choose.
Some types of fast food appeal to me more than others. I'm actually not much of a cold bento box fan in general; I usually end up picking at them at conferences. But I like a lot of Japanese food generally.