You should be as specific as possible. This is a good opportunity for you to show that you have thought a lot about the decision to apply to Columbia and GS.
Talk about what makes a Columbia education THE education for you. Which professors do you want to study with? What is it about Columbia's economics department that makes it your first choice? What about the curriculum?
Further, what specific dimensions of the GS education appeal to you? The Core? [pro-tip: it's not the financial aid.]
Here are some things that you should NEVER do:
1. Do not talk about New York City. There are dozens of other schools in New York. Spending any time talking about NYC is a lot like telling your girlfriend that you love the fact that she's a girl. It goes without saying.
Of course, this isn't a hard and fast rule. If you're applying, for example, as an art history major, the way that Columbia has collaborated with museums across the city literally gives you a Columbia and NYC specific advantage that no other school can offer.
2. Don't say this like, "...because it's Columbia! You know?" I know you wouldn't write something just like that. But, people do often make the mistake of calling attention to brand power. A lot of schools have it and, like New York City, it's best left unsaid.