Having spent my life looking for patterns, I'm ashamed of how long it took me to see these two. The first one hit me only yesterday. Americans (including me) will be voting for the member of a nuclear "family" they want to lead them: Dad (McCain, the protector), Mom (Hillary, the ambitious hard-worker), or Son (Obama, the futurist)?
Alternatively, today it occurred to me that we're also choosing our favorite century: 19th (McCain), 20th (Hillary), or 21st (Obama).
Looking at it either way, you can see why I haven't decided yet. If I'm in a pessimistic or fearful mood, I like McCain, pragmatic - Hillary, optimistic - Obama.