It seems horde wins 80% of AB's, 90% of WSG and 30% of AV's in the battles I've participated in. Alliance side, it feels like the horde win far more then they lose.

But it depends on queue times too. AB/WSG are very quick queues and AV is usually 8-10 mins a match, but from an alliance point of view, AV is the battleground to be in.

Eventually, I'll be gearing 1x Disc/4x Affliction Warlocks (horde) and 5x Balance Druids (alliance), both on Cenarius, which is Whirlwind; will get to see the server/battlegroup from both sides.