The focus must be on the key question word in a question, so it must either follow は, which marks the topic / already known information ( 南 は どっちですか) or followed by が, which marks the subject of the comment / new information ( どっち が 南ですか) .

I have read that question words can't be the topic of the sentence; they go after the は.

I was wondering that too... idk

I have the same question. Isn't it grammatically correct?

どっち cannot be a topic so can't use は. You can use が as in どっちが南ですか。

it sounds better the other way, but it's not incorrect, and if you said it in a conversation I would certainly understand what you were saying.

Do you mean どちら？ どっち is sort of a contraction of it.

こっち - this way (close to speaker) そっち - that way (close to listener) あっち - over that way (away from both speaker and listener) どっち - which way (part of a question)

Exactly! I was wondering the same thing.