I went with the Vol because the plot given to me by the sage in Fort Haven suggested that was where we were supposed to go first. So I did.
I have a feeling that the game was made with intent in mind. If your priority is plot, and figuring out what caused the Calamity, you go to the Vol first. If your priority is rebuilding your forts, the Ukat should be first for the iron. And if your priority is getting out of the place and going back home, then the Ahriel is first (For the quicksilver).