Regular decision acceptance rate is 25%. Early decision is 89%. You get first pick of what kind of dorm you want (double, double!!) AND YOU GET TO PICK YOUR CLASSES BEFORE EVERYONE ELSE! This means no waitlists for you! You get first picks! (That is something you really really want!) Yay! I STRONGLY ENCOURAGE EARLY DECISION
All the above is useful info, but anyone who needs financial aid should avoid the ED route so financial packages from all schools can be compared simultaneously. Let’s say you get accepted to American ED with a financial aid package that could work, but only with significant loans, parental penny pinching and sacrifice, a lot of work-study hours, etc. Do you take it? Do you turn it down in hopes you get better FA from another school during the RD process? Better not to have to make that decision. Unless you are lucky enough to be full pay, your FA situation is a lot more critical than getting first pick of dorms or classes.
The regular decision rate was actually 23% 
so it is even (slightly) harder to get in RD
Do you think ED II would help boost chances as well, or is that not as helpful?
American University values interest highly. Open every email, click on every link, attend every webinar, go to the admissions session when it comes to your high school. Get to know your admissions counselor.
One should only apply ED if money is absolutely not a concern. American will throw money at highly qualified students it wants, but it can be very stingy with students it accepts whose stats are in the “middle 50%”.