Farsi (Persian) or Arabic?

I'm an International Affairs major, and I've already got two semesters of Farsi under my belt. However, my professor is switching to all-Farsi next semester, and I still don't feel comfortable enough in the language. The catch: I need four semesters of a language, and I want to work in the middle east after grad school. As a non-native speaker of either language, with fluency in spanish, what would some of the language buffs recommend? *As a side note, I am aware or the syntax-ical similarities of the language, so I'm looking more from a difficulty to maintain or learn angle. Thanks!