42.0 US or America
From Inaugural Americans
from Language Log by Mark Liberman
more at http://languagelog.ldc.upenn.edu/nll/
In a comment on my post about relative word frequencies in the vice-presidential debate, Roo suggested that there's "a difference in mindset/strategy between conservative and liberal politicians", where conservatives tend to use "America" while liberals use "United States". While this was true in that debate, I'm not sure whether it's true in general. As a start towards addressing the question, I took a quick look at the frequency of words based on the morpheme America (e.g. America, American, Americans) in the repository of inaugural addresses at the American Presidency Project.
The results show an overall rising trend, but no clear conservative/liberal division (at least none that's clear to me)