Primarily, it's a gut feeling. I rely on those a little more than I should, but I'm also not the most experienced SE player. (Yes, I know you all said I can't play the "new player" card after my first game, but...here we are.)
I'm fairly certain that the Szeth train last cycle had at least a little bit of Elim corruption, and of the options, to me, Fifth seems like the best one to vote for.