I think the difference between electing McCain or Obama is the difference between playing Russian Roulette with a semi-automatic pistol (McCain) and a revolver which is loaded with 3 of its 6 bullets.
In detail this means that electing Obama is a 50/50 chance to either save the nation or "destroying" it completely, while electing McCain looks pretty clearly like suicide with the only chance to "jam the gun" instead of killing "yourself" ... although this chance is far to low to hope for.