Since Rand is no longer with us, we can only guess what she might have done. Here I offer merely two possible conjectures: (1) Either she would not have voted for either candidate; or (2) She would have voted for McCain. Let's examine both possibilities.
(1) The evidence supporting the conjecture that she would have refused to vote for either candidate comes largely from her refusal to vote for Ronald Reagan in 1980 and the fact that her intellectual heir, Leonard Peikoff, has stated quite clearly he believes that the Republican Party should either be wiped out or "severly punished." Peikoff's reason are stated in a recent podcast:
Peikoff's views are clearly influenced by Rand's disgust at Reagan's alliance with the religious right. The fact that the Reagan needed this alliance in order to have a chance at winning is of no concern either to Rand or Peikoff. In Peikoff's case, this militant animus against religion is taken to the length of insane paranoia. Peikoff mocks Palin for some of her odd religious beliefs, yet he fails to cite any instance in which her personal religious convictions have affected her political decisions as governor of Alaska. Yet Peikoff's paranoid horror of religion has its source in Rand, so it is quite possible that Rand would agreed with Peikoff and would therefore sit out the '08 election.
2. The evidence suggesting the possibility that Rand might have voted for McCain largely comes from her fear of George McGovern in 1972. Obama is the most left-wing candidate since McGovern: indeed, he might even be further to the left. Like McGovern, Obama is an anti-war leftist who supports a very strong redistributionist agenda, including giving so-called "tax credits" to people who don't pay any federal income tax. Would Obama scare Rand enough to make her vote for McCain? After all, even Peikoff admits that Obama is the first major Presidential candidate to have anti-American views. If McGovern was scary enough to cause Rand to vote for a President who extended the Great Society and implemented price controls, why wouldn't Obama be scary enough to convince her to vote for McCain?