Vote for Fred. First of all, the more support he gets, the more he gets noticed. Second, SC is sort of do-or-die for him - he's put a lot of time, money and effort into SC, and if he doesn't do well then a lot of people will write him off. This can only help McStain, as many of Fred's more national defense-oriented supporters will look to someone with strong credentials in that area.
Second, the fewer votes that Gomer Huckster gets in the deep South, the faster he'll implode. When he does, then a LOT of his voters will go to Fred.
McStain won't, IMHO, get the nomination - he's torched the base too many times. He's actually less of a danger than Huckster, simply because Huckster takes almost directly from Fred. Without Huckster, Fred would be far ahead of where he is right now.
Uh, THIRD.