The Horde rules in PvP.  Better PvP racial traits.  Better PvP players and better teamwork.  If you think you may like to win 75% of the time in PvP battlegrouds, go Horde.  Otherwise, the rest makes no difference.

I play Alliance and like everything about Alliance except for 2 things.  1.  We don't have a decent quest reward fishing pole (we have to open a ton of underwater traps to get one or pay lots of gold)  2.  We routinely gets spanked in AB and WG, so if we're collecting marks for pvp rewards, we are in for a long journey of punishment.

I'm sure you can find a good guild in either faction.