One Big... Thing. I do believe the U.S. tried the whole "smaller nations working together" thing once, under the Articles of Confederation. It turned out horribly. Like a bunch of smaller U.S s, with a lot less resources, and all wanting more. Unity is, quite simply, in everyone's best interest. I vote for one big thing.
Of course there is. I live in one of the northern states. Michigan, actually. In a little place called Detroit. Trust me, southern Canada is one thing, northern U.S. is another. Besides, southern Canada IS Canada. The vast majority of Canada's population, something like 90%, lives in southern Canada, along it border with the U.S.