Timmins is probably first pick for the nickel ore processing with no matter which road gets built first. I base that most on the existence of the Kidd Creek mine in Timmins. Aquiring it might be a cost saving and certainly much time saved.

But if the Marten's road and followed by railway is built then my guess is that Sault is the one to get the nod for the chromite smelter.
For Thunder Bay to get the chromite smelter they would have to lean onto the native communities along the E-W route for them to get their act together and pronto. Without E-W route it looks difficult for Thunder Bay.

Of course politics have a lot to do with a decision and not just costs and practicality.
I have no particular insight, just my opinionated judgment. Cheers, Ed.