Since you're putting it that way, in the form of a hypothetical yet improbable (due to current two-party system rules) three-way election, I think the republican vote would simply be split between Romney and Cain, while the full measure of the Democrat vote would go to Obama.
On the other hand, let's assume Clinton and Obama both ran against Cain and Romney. Who might win then?
Haven't a clue.
Getting back to reality, I think it's important to work within the actual structure, and doing so, my original point stands in that I believe people need to determine what's most important to them. Is it the election of their candidate of choice? Or is it to ensure their party's candidate gets elected?
That's where things get complicated, and depending on one's goals, one's choice may be dependent on the candidates in the other party.