By staying there, you will have demonstrated your loyalty and your belief in the strength of the firm. All you have to do is continue to produce a top quality work product and ask for more responsibility and, more than likely, you will be pleased at the result. But even more important is the big picture. I understand that the branch office is part of a firm that is very strong in its particular geographical region. However, the firm you are currently with is not only greatly respected in New York but is highly regarded nationally as well as internationally.
Branch offices can be risky business.
Up until this partner joined, the litigation department in New York has been on the small side and certainly not a major player in New York. It would be wonderful if this particular partner was able to turn the litigation department into a powerhouse but that is a long process and the odds are against it. I do believe that clients hire lawyers, not law firms, but sometimes there are certain extenuating circumstances that affect their decision to follow their primary attorney to a new firm. These extenuating circumstances are very relevant in your particular situation. This departing partner's new firm is trying to make a splash in New York and, even though they have made some inroads, they are still not considered to be among the one of the main firms to choose from in New York.