Is there REALLY a difference? I started with Alliance. I did PvP a few years after initial release, love it. ALWAYS played Human and Night Elf. I was tank \ healer. So very recently (like 6 moths) I moved ALL my toons to horde, even leveled a Mage as horde..
As Alliance I could barely stay 50 \ 50 for BG wins.. yeah maybe I suck, maybe it's me, maybe I need to learn how to play melee. Overnight literally the very next week my BG win percentage was roughly 80% (up from 50% ish...), and it has not gone down since. Dramatic improvement. I actually have winning 2's \ 3's in Arena.. it's barely above 50% but it's a positive none the less.
What's my point? How can be alliance to barely winning random BG, never hardly won any Arena and the VERY next week as Horde I win more, and for the first time my PvP rating means I finally don't feel ashamed..