Totally different headphones but If you can return them, get the 400i, they are noticeably better. If not, then just enjoy the 400s as they're pretty awesome too.
I'd want to keep them both. If there is a noticeable difference I'd pick up a pair as the price is now the same. I'm not too fond of the headphone connectors on the 400i. I like those on the 400S.