I got the admission both AU(The American University Washington College of the Law) in International Business Law and Indiana U-Bloomington.
I heard that AU has a good reputation in international law but IU's ranking is higher than AU. (IU-36/AU-47)
Is ranking important?
Which one is your best choice for LLM program?

It depends on what you are ultimately interested in.
American University has a top notch reputation for international law, which is what you would be doing your LL.M. in. In this area, AU is regarded far above Indiana.

You mentioned the general US News rankings. Those rankings pertain to the reputation of the school's JD program specifically. Moreover, if you are concertned with rankings, look at the US News rankings of "International Law" programs. American University is ranked in the top 5, along with NYU, Columbia, Harvard, Yale and Georgetown. If you are interested in studying international law, AU has a far stronger reputation in that area than IU.

As I noted in my other post, in terms of job prospects, the legal job market in the US is quite poor. I am not sure if you are an international student, however, if so it will be that much more difficult to find a position in the US, regardless of the LL.M. program you select. With that said, however, it would be a lot easier for you to find an internship, which may lead to a job, in Washington, DC, simply because you would be in the capital of the US which offers a plethora of opportunities.