Obama definitely. McCain himself probably wouldn't be so bad either if he were younger and more capable to be himself instead of what the Republican strategists tell him to be. The ultimate dealbreaker however is Mrs. Sarah "Pitbull with a lipstick" Palin. The opinion on that woman went from "wow she's hot" to "wow she's dumb" unbelievably fast, and some of the things she said are actually quite scary. Electing McCain means a significant chance of Palin actually becoming President, which is something that must never, ever happen. Bush might be generally perceived as dumb, but this woman takes the cake in that regard and she didn't need years to make that painfully apparent.
Besides that I think that Obama might actually change something to the better in the good ol' US of A, which frankly in the long run has probably more of an impact on us than local elections here (considering how much the USA like to play world police and "solve" conflicts by arming their future enemies).