Sets dependent on weather are a big no-no, because of TTar. And unfortunately, our FireFox, I mean fiery fox does nothing to it, barring HP Fight....
I would go with HP Grnd, since atleast it scores some Super effective damage on TTAR.
But Ttar hates Will-O-Wisp. In the event of Ttar and Heatran at the same time, I'd just HP Ground until Heatran is out of the way. Anyway, Ninetales shouldn't really attempt to take on either of those two.