If I were you I would choose Turkish as it would bring a wider perspective considering that Turkey has been a bridge between the east and the west. Furthermore, Turkish will probably be one of the official languages of the European Union in ten years time, in fact, it will be the most widely spoken language of the union by that time.
Considering the number of people that speak Arabic, Arabic might seem like a good choice, but as a person who has some knowledge of the language, I have to tell you that it is not possible for a Lebanese Arab to speak with a Yemeni. The text would be comprehensible for both, but dialects are unbelievably different. As a native Turkish speaker, I can speak Azeri because the two languages are like American and British English. Due to the closeness in the language tree, I can easily communicate with an Uzbek, Turkmen or Tatar. I do somewhat understand Kazakhs and Kyrgyzs. Besides these, please see the wikipedia link below.
According to the UN, among the languages that use Latin alphabet, Turkish ranks 4th, below English, Spanish and Portuguese, respectively and above French, German and Italian. According to that statistic 99,866,460 speak Turkish as their native language and 30,381,893 speak it as their second language. Besides Turkey, some of the statistics for the speakers of Turkish are as follows: Iran (13.5m), Germany (3m), Russia (10.4m), Afghanistan, Bulgaria, Saudi Arabia (0.4m), the Netherlands (0.23m). Don't forget these statistics do not include Turkic languages, of which most are intercommunicable.
Last but not least: the ease of learning a language. Arabic is one of the most difficult languages to excel at not because of the complex script, but because of the large number of words that are necessary to speak the language. Arabic is the richest language in terms of the number of words. On the other hand, at a daily level Turkish can be spoken with only 1200 words. There are many loan words from western languages especially English and French that could help you. Turkish an agglutinative language. If you don't know the meaning, don't worry. This means that a root can be used to create words by adding suffixes. An easy example:
öğret- (verb: to teach)
öğret-men (noun: teacher)
öğret-men-lik (noun: being a teacher)
göz (noun: eye)
göz-lük (noun: glasses)
göz-lük-çü (noun: optician/person who sells glasses)
So, with one root and a few number of suffixes you can learn tens of words without effort. :)
Fınally, Turkish is extremely systematic in terms of its grammar. As I studied German as my third language, it was shocking that I had to memorize an article (Artikel) for EACH word because half of the dictionary does not follow the general rules. With regards to pronunciation, I can teach any person speaking a western language, to read Turkish perfectly in less than an hour because there are virtually no pronunciation rules as a word is pronounced as it is written. It's enough to learn the alphabet.
I hope that has been of help. Feel free to contact me.