I want unpredictability. Not only in the type of ending, but what happens in it. Sad endings are more unpredictable, if only because they're rare in the first place. EDIT i don't really mean "everything dies", more like "It isn't a happy victory by any means". Simple destruction is too predictable, and boring.
Besides, Halo isn't the kind of world that should have a happy ending. I think most of its endings have been too happy so far. It is the kind of world where a good ending should require far, far more work than its endings so far required.
And bittersweet ones... well, none of Halos bittersweet ones really work so far, IMO. Halo 4 especially is bad in this regard. A piece of software "dies", a blank character feels sad, and a million is a statistic.
EDIT not that predictability is necessarily a bad thing, but if something is predictable, it should be also damn good while at it. It is easy to predict what happens in Mass Effect 2 if you ignore every side quest and ship upgrades... but the ending works well even then. And it is easy to predict how it goes if you do everything, but it is also awesome then.