since you’ve played all alliance i would go with tauren. Gives you something new to expirience

Both of the above answers are correct in that whilst the race may differ, you are essentially changing very little about your character. The largets variation will simply be in the visuals of your characteron the screen, and becuase one is Horde and one Alliance, the item drops and mobs may also differ during the opening stages of character leveling.
As it’s been said, it would be fun to experiance somthing new for a change, so if your an Alliance veteran, try out the Horde and see what batting for the dark side has to offer!
If you want to get technical, then you need to consider the racial traits of your character, as this will have a minute impact upon game play style. Both Taurens and Humans are considered the traditional tanker warrior race of each faction respectivly, so its not as if your choosing between a tauren and a nightelf whos racial abilities are more inclined towards fleeing or stealthing (as with shadowmeld an dteh increased spirit form speed).
Check this site to learn more, and for a quick run down, here are the important stats of each of your prospective races.

Human Base stats: humans are less strong, but more agile in its place. They also have less stamina, but are more intelligent insted and are fairly similar to tuarens in spirit.

With this in mind, make your choice and most importantly….
HAVE FUN!

(*btw I would personly choose a human simply becuase I enjoy there strating area better) ;D