Sapphie - in addition to Pascal's comment - the difference is in the topic/object 을/은 vs. subject 이 marker! That means that as you see, the "것" (thing) in question sometimes is the subject and sometimes the object of the sentence. Which might have consequences, but mostly doesn't. And most people use 이걸 or 이건 anyways. I can't exhaustively say what that means, as in this case "뭐예요" is the most simple sentence construction possible.
But if you imagine a sentence such as "소리가 진짜 많아요! Sophie씨 이것은/을 뭐예요?!" you can already see the reason for both options. Because Sophie is the subject of the sentence (I just assume that now - correct me if I'm wrong) and "소리" is the topic of the (previous) sentence / conversation, we use 은/을. It's a basic concept, maybe you know it already, which would be great if you do. :-)
PS: I hope you noticed your misspelling of "예" as "여", which is what Pascal means by "dialect".
