For U.S. families, if it is possible for you to come to Haiti to file your I-600 in Haiti at the Port-au-Prince (PAP) USCIS, then do so.
If the I-600 is filed in Haiti, then the orphan investigation (I-604 portion of the I-600 process) is conducted by USCIS in PAP. That office is efficient, polite, user-friendly and professional.
If the I-600 is filed in the U.S. then the U.S. Consulate office is responsible for the orphanage investigation. Unfortunately, since October 2007, that process has been slow and stressful on the adoptive families, the adopted children and the adoption professionals (contrary to what the Foreign Services Manual tells U.S. Consulate workers to do).
Though I have read some of the praise-stories of a few families on Haitian Angels about the kind treatment they received at the U.S. Consulate, the majority of families have not had that experience.
It will take less time to process the visa portion after the adoption is finalized if you file the I-600 in Haiti.
Also, if you can, you should participate in the Adjudication Orphan First Pilot Program (AOFP) because then the orphan investigation is done before the adoption is finalized. However, you must file that request in Haiti because if you file that request at your local USCIS office in the U.S. chances are that they will "mess up" the request. (It happened in my adoption.)