Depending on your location some mirrors will be faster than others and some might even have an unstable connnection. For example I come from Germany (Berlin region), so ftp.de.debian.org should be best in my case. But in fact ftp.cz.debian.org is usually faster and more stable for me. So if you live closer to New York than to Indianapolis (that's where debian.org is registered) in terms of server hops, usually also but not necessarily geographically chosing the Columbia University mirror would make sense.
EDIT: Its running now.