Dana Millbank wasn't born yesterday, but I have to say that this question is remarkably naive: "[W]hich one will Romney choose: defense spending or tax cuts?"
The obvious answer is: neither. A President Romney, with a Republican Congress, would almost certainly choose very large deficits rather than cut defense spending or raise taxes. After all, that's been the policy of incoming Republican presidents beginning with Ronald Reagan, hasn't it been? Eat dessert now in the form of enormous tax cuts and spending on GOP priorities, and then remember the overriding importance of the deficit later on, preferably when the Democrats take over.
That's what Reagan did. That's what George W. Bush did. The only exception was George H.W. Bush, who was a real deficit-cutter. And he wound up repudiating it when conservatives revolted.
Now, Millbank is surely correct that if it ever gets down to it, Republicans care far more about tax cuts for rich folks than defense spending. And it's certainly true that Democrats are trying to force that choice on them, and that Republican complaints that the sequestration is some sort of insidious liberal plot sort of ignore why it was on the books in the first place -- although Republicans surely are correct that they didn't want the defense portion of sequestration.
Millbanks says that "If Romney wants to keep his vow not to cut Social Security and Medicare for those age 55 and older, he’d need to shut down all functions of the departments of Commerce, Education, Energy, Interior, Justice, Labor and Treasury as well as the National Institutes of Health." Well, that's the true effect of the promises Romney has made, and I think it's definitely fair game to point out the implications of what Romney and House Republicans say that they would do. But in reality, they're not going to shut down most of the government; they're going to blow up the deficit.