Sharp-shinned Hawk

I bought a picture guide to birds, animals and plants of TRNWR. I thought it would be helpful in more quickly identifying difficult birds. Well, I got a tough one today. According to the guide, both Cooper's Hawks and Sharp-shinned Hawks frequent the refuge. The picture above is one or the other. They look very similar. I decided on Sharp-shinned based on two things. One, its tail appears to be squarer than what a Cooper's Hawk is supposed to have. And, two, its head is fairly small.
Cornell's All About Birds website states that these to hawks are some of the most difficult birds to distinguish between and I don't doubt them. The pictures look very similar. And since this is the first of either that I have taken pictures of, I'm very green at making the right call. I'm about 51% sure I made the right call in this case. ;-)