General Zod
Ruler of Australia
Actually, of those you listed, learning all but Chinese over Spanish would be the error in judgment. Spanish is the fourth most widely spoken language in the world.Learning Arabic, Chinese, or French is a mark of intelligence. Learning Spanish is an error in judgement.
http://www.infoplease.com/ipa/A0775272.html
That leaves us with Hindustani and Chinese. China's markets are difficult for American companies to get a foothold in. Consequently, learning Chinese for business would currently have little value. After the U.S. and the U.K., India ranks third in having the most English speakers. So, there's little business advantage to learning their language, when they're already proficient in ours. Additionally, a lot more business is being outsourced to Latin America, because of the timezone issues and the rising costs of doing business with India. From a business perspective, I'd rather be fluent in Spanish.
Mexico shares a continent with us. And whether we like it or not, there's a lot of Spanish being spoken here. So, from the perspectives of practicality and proximity, I'd rather be fluent in Spanish.
When vacationing/traveling/retiring, I'd much rather do so in various Latin American destinations; Mexico, Panama, Costa Rico, etc. So, from the perspective of someone who travels and would love to retire some place warm and beautiful, I'd rather be fluent in Spanish.
If you feel choosing Arabic over Spanish, bares the marks of intelligence and insightful judgment, I respectfully disagree.
Zod