Gingrich and Romney are both just political operators who will say anything they can to get elected. Gingrich is better at it, but Romney has better funding, organization, and party backing.
I think Santorum and Paul both genuinely believe what they say (Paul especially) but I dislike them as candidates for various reasons. As a practical matter, they're both too far right on social (Santorum) and/or fiscal (Paul) issues to succeed in a general election.
I think it's pretty obvious that the Republicans are running their second string right now. Say what you want about Obama, but he's definitely a formidable campaigner and the GOP knows that. I think they're saving their real candidates for 2016.