The southern route would make sense as the slowed down crossing only takes six days, Cunard, I believe, changed from a 5 to 6 day crossing to conserve fuel and to be able to charge more for the crossing. In January the weather in the Atlantic can play up so I think you have sussed it out well but for the definitive answer why not try the horses mouth....Nei.
Two points. It is actaully 8 nights, she arrives in New York on 10 January 2009. I am travelling with her as far as Ft.Lauderdale to pick up The QM2. The extra time will be due to Queen Victoria not being a liner, she is to all intense and purposes a Cruise ship and this is the first leg of her World Cruise. See you on her.