I agree, great discussion, and one of my favorite group of birds!  

A quick follow up regarding the streaking pattern. As Ken was mentioning, on the goshawk, the streaking continues all the way down into the undertail coverts, whereas with the Cooper, the streaking continues all the way down the belly, to the legs, and a bit beyond, but then stops, does not continue into the undertail covers, which appear clean and white.  

My previous comment on the streaking was focussed on the pattern itself, that I don’t trust using the shape of the streaking, or the color, to help identify Cooper versus sharpie versus goshawk. That became apparent to me when I was seeing these birds in my yard, and this picture shows two very different presentations of a juvenile Cooper. At that point I stopped using streaking pattern itself as a way to distinguish these birds!