I play both, and the the only real thing that I think would give someone a reason to play alliance over horde is if they're just naturally inclined to the blue good guys rather than the red bad ones. Horde has received more attention ever since that vanila beta patch that focused heavily on the Horde (the one that moved BFD into Ashenvale, and put in Zoram's Strand etc...) and quite frankly, the playerbase is full of less mouth breathers. The differences in pugging is night and day. The Horde characters aren't necessarily more interesting characters, but they are vastly better utilized.