PvP server and If you want to World PvP Horde may be the best choice (LFH a must) as they can go to Redridge and gank questgoers only in their teens, alliance has no real good ganking area for a lvl 20 but I suppose ashenvale and hillsbrad will do, thing is those are both 20-25 zones, redridge is 15-20 so a geared trial Horde (any class) can go to redridge and feel like those 24's feel, like a dick.
Alliance hunters and rogues may not be as disadvantaged in terms of Wpvp in ashenvale/hillsbrad, probably why theres so many of them, that and the eyepatch... Who can resist rolling alliance? they get eyepatches!