I would do the pullups if the reps are about the same. I do the same thing, when it called for the close-grip pulldowns, I sub close-grip pullups. I wasn't able to get the same number of reps in, so I did one for set at the end and then the next time I do it I will do the pulldowns. It can't hurt to switch them up, but the pulldowns will help you do more pullups.
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