After a second, longer look at both 1558 and 1560, I can see your point about the more rounded head. However, because of the angle of the shot, I'm not sure I'd rely on that as being definitive in this case. Unless you recall that the bird did in fact have the less helmeted look (of a Cooper's) when you were observing it? Frequently, your first, live impression of a bird is the most accurate.
Also, the tail shape referred to in the books is based on the fanned, not folded, tail, so unless you can get a clear enough look at the tail itself to see the shorter, outer feathers (which would indicate a rounder tail), I don't think these photos are definitive there, either. In a tightly folded tail, those feathers would only be visible from a front view since they are tucked on the underside when the tail's closed. Sometimes when the tail's less tightly closed, you can see a hint of the shorter feathers on either side even when viewed from the back.
I.e., I honestly don't think we can tell from these photos whether this is a Sharpie or Cooper's -- unless you could hazard a guesstimate on the size of this bird? Sharpies are 1/3 smaller than Cooper's (roughly 11" vs. 16" respectively).
And don't worry if you don't feel definite enough about an ID. There are a lot of birds that just have to go un-ID'd for whatever reason. I see a Sharpie almost daily in my yard, Cooper's much less frequently, and I wouldn't be able to call this one of yours with much confidence. Its bulk in some shots makes me think Cooper's, but then in others, it has the more sleek lines of Sharpie.
BTW, give my regards to Modesto -- my hometown (but only 'til I was 2).