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For N/24 calculation or in general?

You should get the same N/24 results from using either, it's just more work doing it at CK.

Except that CK will give you open dates down to the day, while Experian only gives you the month on their website. But if you're not cutting it really close, that doesn't matter much.

For certain other things, CK is more useful, including TU and EQ pulls, and free credit scores, so definitely if you're interested in more than just 5/24 calculation, you might as well sign up for both. And there's no harm in doing the N/24 calculation at both of them, but if you get a different result, double and triple and quadruple check, because you shouldn't get a different result.

Things like inquiries are different at each of the 3 credit bureaus. But the card account info should be pretty much the same at all 3 credit bureaus, unless one of them made a mistake.