Sertorius will push for the Sixth in-story regardless of your decision, but I'll only actually roll for it if you say yes. If not, he's automatically shot down and given a different set of legions to take Spain.
It's one of those weird votes where the players themselves have more power than Atellus does in order to allow the conceit of choice. Those will get rarer as time goes on, but I figured I shouldn't send you all off to Spain without a choice in the matter.
No, because I already wrote all the important bits in the update except the parts you all pick.