This certainly depends on the school which you attend! However, from my experience, AP US History is much more difficult. It requires a lot of time spend pouring over the text outside of class and the exams are very difficult. It's often primarily a rote memory based class. However, as previously stated, you can take the AP US History exam which can allow you to avoid a college class. That being said, the exam is often extremely difficult and sometime the class doesn't cover everything that is on the exam. When I took the exam, the essay section was about the Korean War and we had never gotten to that in my class, so be careful when you decide whether or not to take the exam and study a lot.
Global Studies 2 Honors would probably push you in different ways and would give you a better view of the world around you which is very valuable for applying for colleges and jobs. However, there's not an opportunity to take any AP tests and test out of college classes (unless your college offers some sort of "CLEP" test which can also exempt you from classes).
AP US History is an indepth study of American history. It requires not only memorization of facts, but also critical thinking and the ability to apply facts learned to important historical concepts. Though it is a difficult class, it is very rewarding, especially since there is a chance to earn college credit (which would exempt you from a college class) by taking the AP US History exam. Global Studies 2 honors is also a very challenging course, but it has no prospect of earning college credit.