Look out! and Watch out! mean the same thing this way. An example is if something is about to hit your friend you can tell them to look out, or watch out! Or if your friend is about to step on something. It can be something to quickly shout at someone. But you could use these words differently. For example, "Look out for yourself." This can mean take care of yourself. But you can not say "Watch out for yourself." If there some bad people in the neighborhood and your friend is going for a walk outside you could say, "Watch out for those guys" To tell your friend he or she should pay attention and be careful. Look out could also be used but usually watch out is used if it is being careful or paying attention to something negative or bad. But "look out" could be used as well. I guess it is a personal preference.

I don't think there is any difference, but people in different countries may use one more often than the other.