I played one year as Alliance and three years as Horde.
From a lore point of view I would be the Alliance - although, never a human. (Those guys were such bastards.) - because their lore suits me better, but in the World of Warcraft universe I'm definitely a proud member of the Horde.
From my experiences on the realms I have been on Horde have been generally friendly and nicer players than the Alliance. The Alliance seemed to be full of immature douchebags who just enjoyed ganking people. This is definitely also true for the Horde as well, but I find the Alliance do it more.
For the record: I played Night Elf Druid, Draenei Priest and then Blood Elf Mage and Tauren Druid.