Yeah exactly what H2OfowlND said. Last year we were there the last full week of October. I bought back a Drake bufflehead that was is great plummage and we got others that were as well (widgeons, redhead, canvasback). We never did see any nice shovelers in full plummage but from what I know these guys get full color later then most of the other birds.
From what I have read and heard if solely going for wall hangers, the later in the season the better so down south would be my pick for that. With that being said, I have not made it down there yet and love hunting in ND and heading back again this year beginning of November.