As a liberal I have no problem with Mormonism even if it seems cranky and heretical to me. I have a problem with religion in politics when people shamelessly exploit it in an 'Believe it or burn' mode on politics.
Anyone who votes for President Obama because Mitt Romney is a Mormon or because Rick Santorum is a Catholic votes for the President for the wrong reasons. We need to ask what is good for America instead of what is comfortable for our view of the world. Now if someone votes for President Obama because either Mitt Romney or Rick Santorum offers no viable solutions, then one has a valid cause for voting for the President.
Of course I would never vote for a $cientologist (a fundamentally dishonest world-view), let alone a Satanist.
So if a Mormon were a nominee and you believed they were the best person and the most qualified to be President, you would vote for them even though you believed their faith to be cranky and heretical?
A suitable politician must show competence if an incumbent; a series of offices means nothing if one is a pathological liar, contradicts himself frequently, has a history of corruption, or shows despotic tendencies, or otherwise shows himself unfit for the office I wouldn't vote for any politician who puts the promotion of his religious views over all other matters of public policy.
I'm not sure that anyone can quite say what makes the "better person" and that anything in particular makes one more qualified. A record of solid achievements for an incumbents Could I vote for Harry Reid? Sure. He is a Mormon. Character trumps creed, and someone who touts his creed had better live up to the benign parts of that creed.
For most of us it comes down to ideological concord with us -- right?