Arabic is spoken in a larger number of countries and would therefore offer more flexibility in terms of projects.
Arabic will allow you more opportunities. You may even get a hiring bonus for having that language skill, depending on your chosen career path!
Be sure to study abroad. There are scholarships to pursue language study of Arabic that can finance your study abroad. Check out CLS (http://www.clscholarship.org/) and Boren (https://borenawards.org/) for a couple of examples.
I suggest Arabic. There are around 300 million Arabic speakers around the world; on the other hand, there are only 100 million Farsi speakers around the world. Moreover, Farsi is the official language of only three countries where Arabic is the official language of 28 countries if not more. Lastly, Arabic is the 3nd most spoken language around the world.
I am sure if you do more research you would find even more reasons to go with Arabic.
I strongly suggest you to learn Arabic because it is the official language throughout the Arab world which includes 22 countries, with a population of over 480 milions, that gives you more opportunities and also to hold important and positions in International law