Boxing or kickboxing, which is more effective for street fights?

Whichever one teaches you to fight for self defense is the one you should choose. It's all about the instructors, regardless of what people say. If the instructor is crap, the student will be crap. It's not so much about style as it is about learning realistic self defense. If both instructors are good, I say choose the cheaper one, which will probably end up being boxing since you would have to buy less gear (if you have to buy gear).