I don't think that question can be answered just as easy as that. Every team setup you counter will have con's and pro's no matter what both of you spec.
I'm not exactly the best PVPer in all of ze lands, but I'm running shadowfrost now (0/19/52) and that works pretty well. More focused on spells and diseases, diseases ticking for 800-1000 + your warlocks DOTs should prove to be quite awesome coupled with the burst you will have as well.