Newt Gingrich is an exciting candidate and could make an excellent president, but his admitted moral failings as well as his inability to maintain the support of his own caucus in the late 1990s demonstrate that he is too flawed. Even during this campaign he has sometimes seemed a bit too unpredictable, such as when he attacked Romney from the left over Romney's former career.
Mitt Romney impresses me greatly. Don't forget that he was the conservative choice in 2008 versus John McCain (I looked it up.) He has been a steady candidate and he has lived a model life. But his conservative credentials are much weaker than Santorum's, so I choose Santorum. It definitely looks likely that Romney will be the eventual nominee and if that is the case I will back him strongly against President Obama in the general election.