it's probably somewhere along 40/50 at level 40+ right now in favor of the horde..
and exponentially less allies at 60. The horde BG queue is already so long right now that people are actually ganking low level alliance players around level 51-59. This is all the PvP they will ever get, if not crushed by a premade ally team they waited an hour to meet.
And this is the direction the server is going, so if you plan to do any PvP choose alliance.
Now don't tell me you weren't warned. Get instant queues on alliance side or wait for hours on horde just to face premades.