If you choose Arabic, you will almost certainly end up studying MSA (Modern Standard Arabic). Be aware that no one actually SPEAKS this version of Arabic, although they do write it. Colloquial Arabic differs a lot, in both accent and vocabulary, from country to country. Most everyone can understand Egyptian colloquial Arabic, because Egypt has the Arabic equivalent of Bollywood, so Arabic-speaking people everywhere become accustomed to Egyptian vernacular due to the movies. But if you study Egyptian Arabic and go to Morocco, others may understand you; that does not mean you will understand them!
How big a problem this is depends on who you talk to. One of my friends is Moroccan; he could not understand, or be understood, for a few weeks in Egypt. He then managed to sort things out. However, he is a natural linguist. Although he did not start studying English until he was in high school, when he learned it in the classroom, he is virtually indistinguishable from a native speaker now, in both his verbal and written communications. So his ability to slide from one version of Arabic to another is not necessarily typical.
However, here is a website with some background that suggests the problem is not as insurmountable as it is made to seem.
Arabic is fascinating but requires a LOT of hard work. Not only is the written alphabet different, but there are other difficulties to contend with as well: vowels are usually left out, and letters are written differently depending on whether they appear as the first, middle, or last letter of a word. Finally, Arabic lettering is considered art, so even after you master the basics reading street signs can be difficult because of the playfulness with which many things are written (imagine that you struggle to read English, and everything you are trying to comprehend is written in Gothic font; it is something akin to that experience).
One language to add to your list of the ones under consideration is Bahasa Melayu/Indonesia (the national language of Malasia/Indonesia, commonly referred to as “Bahasa” although technically that is incorrect as the original meaning of “bahasa” is simply "language). It is relatively easy to learn, as it uses the same alphabet as English; spellings and pronunciation usually match up extremely well; and there are no tenses or cases to worry about (for example, the words “I, me, my, mine” are all one word).
Career-wise, Indonesia is the 4th largest country in the world and the largest Moslem one, while Malaysia has experienced economic success disproportionate to its modest size. There are many business opportunities in Indonesia; the size of its domestic market has shielded it somewhat from the effects of global economic downturns.
Bahasa Melayu and Bahasa Indonesia are extremely similar and completely mutually intelligible. The differences are perhaps slightly more extreme than between British and American English, but not much.
ETA: I just saw even sven’s post and I think she is too harsh. I always wanted to learn an “interesting” language and Spanish, given its relative similarity to English, never qualified in my book. There is nothing wrong with choosing a language because the linguistics interest you. As for me, I was bored by French, and yet I had so much fun learning Indonesian! It is an inflected language (I won’t define that term here but you can easily find out more by Googling), which means it operates under some fundamentally different rules quite unlike English as well as unlike most Romance languages (yes, I do know English is technically not a Romance language, but there are times when it sure seems like one). If you like “the idea” of language study, this is a huge motivation.