In one important way, a libertarian should never describe Santorum and Goldwater in the same sentence: the association improves Santorum, but only diminishes Goldwater.
Tim Stanley @ The Telegraph describes (accurately) the fundamental differences between the two:
Goldwater was a small government conservative who believed that social evils are something individuals have to suffer if they want to stay free. Santorum is a big government conservative who will – with some reluctance – use the power of the modern liberal state to do good where it must be done.
See, ‘Rick Santorum vs Barry Goldwater: why libertarianism won’t win in 2012’.
One should quickly note that Santorum will use the power of the state to do good as he sees it, a view incompatible with libertarianism both in means and ends.
But there is a way in which some (although not Stanley) think they are alike. Many believe that Santorum would lose as overwhelmingly as Goldwater did. That’s probably true
Yet, there would be a fundamental difference: Goldwater’s campaign loss inspired a GOP move toward a small-government message in a way that one doubts a Santorum loss would push the GOP closer toward big-government conservatism.
On the contrary, a Santorum defeat would only remind Republicans about what they came to admire in Goldwater’s legacy.