It's clearly worse for someone who wants to be President to not understand the global nuclear balance than to forget a talking point. That the two points get compared shows how silly the political season has become.
I would agree if Cain really didn't know that China has nukes. I'm not so sure that's the case. It was, at the very least, a gaffe and whether it's silly or not, gaffes have always been of significance during presidential election seasons, and not just during this one.
I'm sure you would agree that a candidate not knowing how many States comprise the USA is worse than not knowing if China has nukes, but I don't think anyone seriously believed Obama didn't know there were only 50 States when he made his gaffe.
It is silly and some politicians seem to get more slack for their gaffes than others, but it's a way to keep score and detect wounds and that's what a lot of people like to do during an election period.
Anyone who has had a brain freeze like Perry did (and most of us have) knows that the instantaneous panic they cause only freezes the brain that much more. It can happen to anyone, but for the sake of his struggling campaign, it couldn't happen to Perry.
I'm not sure that his inability to give a relatively flawless performance when one was so important to him, tells us a whole lot about him, but it does seem like it should, and that will be enough to insert doubt in some voters mind about his qualifications. If you were on the fence with him before last night, the gaffe wasn't likely to have pushed you over to his
side of the yard.