But like Kara said earlier, "kata" (extremely polite) "iru" (casual) creates an unbalanced sounding sentence structure anyway. Best use "nihonno-kata" orare-masuka" or "nihonno-kata irasshai-masuka" OR "Nihonjin-iru" or "Nihonjin wa doko."
It appears as if it should be "wa" instead of "ga" regardless of the placement of "doko ni," although at the beginning may have a more formal tone, I'm not sure. As in where might my car be, "Dochira ni watashi no kuruma wa arimasuka?"
In my personal opinion(or feeling), there's hardly any difference between a) and b) in many cases. Nearly perfectly interchangeable. But if you must find something, I'll say "the prior word has a bit more priority".
So in sentence a), the nationality of the object is more important. If the subject of the sentence expresses it in simplest way, he(she) will say "nihon-jin wa?".
While in sentence b), the location/place of the subject is more important for him(her). The simplification in this case is "Doko?"