Goofy poll, but they're both great beers in my opinion. If you could only have one, what would it be? I'd go with black, though white is no slouch. I love the complex nose of Allagash Black, and its smooth chocolate character.
While I would generally prefer darker beers to lighter, as far as Allagash goes I would have to go with White simply because if I wanted a Witbier I feel theirs is pretty much the gold standard.