Here is what Dr. Dunning said (I've edited out a few unrelated items):
So without further ado, I present a series of photos of a rare bird in Florida. My "Florida's Birds" field guide refers to this bird as "accidental".I am afraid that I am going to have to take sides with your friend. You are right that Swainson’s Hawk is extremely rare in Florida and the usual rule is “when you hear hoofbeats, think horses, not zebras” (e.g., more common is more likely). But Red-tailed Hawks tend to have a strongly marked belly band but not the level of dark marking around the breast area that is seen on this bird. Adult Swainson’s on the other hand have a very prominent dark breast in just the area marked on this bird. In addition, my impression from the photos is that this is a slim bird, which one would not expect for a Red-tail and is more consistent with a Swainson’s. They are a long distance migrant (going all the way to Argentina in the winter) and have a slimmer build with “less shoulders” than one would expect in a Red-tail. So I think the rarer bird is more likely here, that it is a Swainson’s in sub-adult plumage.
Dr. Dunning