I am still uncertain whether it is better to file the I600 locally vs. Islamabad?
When one applies locally it takes them a week to decide (varies from city to city). After which you will receive an 171H approval which signals applicant to contact the Immigrant Visa Section in Islamabad and make an appointment for an interview out there. Homeland Security forwards the notice to the National Visa Center in New Hampshire which records and relays the information to Islamabad. See Pakistan in country process for Islamabad application.