The Alliance, for sure. More pleasant, friendly, and welcoming. I love the horde, but there are too many horde players willing to make blanket condemnations of the alliace and alliance players. It's also pretty clear that the horde have become the real aggressors in the conflict since Garrosh took over. With Thrall the factions were essentially ethically the same, but even back then I'd made too much an investment with the Alliance to spend much time as a Horde. I still plan to Rock a Tauren Paladin eventually.