American, if you’re serious about IR, and doing internships in this field. If you’re doing IR but later want to go to law school or an MBA (for example) UCI would be fine. But American’s location absolutely beats Irvine if you want to work for the government, at a think tank, any kind of lobbying, etc.
For a thorough assessment, research internship opportunities available to American University students. Can you contact some alums? As a transfer student, you may not have the time to do a semester abroad program and still graduate in 4 years - but I would also look into what kinds of overseas possibilities are offered by AU.