considering this to be a serious thread, id pick uff if i were to vote, but im not going to vote......because no matter what happens, if any of these books get canceled its obvious it would be the ultimates at this point...unless someone takes over for millar, the book is going the way of the dodo...first captain marvel, (although not as good) and now the ultimates
From a commercial perspective, I see no point in canelling ANY of the Ultimate titles -- and definitely NOT at this point, when none of their story-line potential has been fully optmized yet.
Having said that, it would be easiest to end Ultimate Spider-Man, because there's already a built-in storyline conceit for it -- forcing him to join the Ultimates on his 18th birthday. He only has two major supporting chactacters whose relationship with him *needs* to be resolved, before the series could end, one of whom (Mary Jane) can be easily cut off with an enforced break-up, with the added bonus of seeing Peter attempt to deal with the fallout like a real human being, rather than angsting over it like a self-centered drama-king Dashboard Confessional wannabe. Spidey, the character, would then appear in Ultimates, the series.
Besides, as Hawkeye's example proves, it's possible for an Ultimate hero to have a *relatively* well-adjusted personal life, outside of his world-saving "career", once you allow for the reality of the increased hazards of his/her line of work.