Why would someone pick Romney over Huntsman or the other way around?

I really have a hard time seeing any difference between these two. They both recognize that there are some serious environmental issues but dislike the measures that have been taken about them. They are both kind of ok with gay rights without outright supporting them. They are both Mormon which only 18% of republicans have a problem with. And most importantly, they are the most moderate republicans running and stand to gain perhaps some votes from liberals dissatisfied with Obama. But why is one preferable over the other?