I don't think either one is more "polite" than the other, although "I wouldn't know" does have a slight tone about it which suggests that you think it's strange that the person asking thinks that you would know. It's not as clear-cut as "How would I know?" said in a somewhat incredulous tone, but it might be taken the wrong way. If you simply don't know an answer, I would stick with "I don't know" or, if you wish to be politer "I'm sorry but I don't know".
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