In my view, it's depend on the sentence structure.

As far as I can see there is no difference. there could be a difference in the very advanced level in english, however 99.9% people use these two words in common though "SOMEONE' is used frequently than somebody.

actually, i don't think there is any difference in definition.

没有什么区别，只不过“~one”的形式比“~body”的形式更文雅一点。