It seems to me that a bivy adds warmth to a bag, but I have no real data to back that statement up. The fact that I am around to write this statement would indicate that using the bivy and sleeping bag provided enough warmth to survive... Perhaps someone does have test data acquired under controlled conditions.

I have no direct experience with the Baffin Bivy, but that is exactly the sort of thing I have used in the past.

I would recommend getting a down bag first, but either will work. As recommended, getting a synthetic bag sized to fit over the down will allow you to use the two in combination.