Horde shammys are generally preffered because you get 2 options. I don't play as alliance so I can only speak from my experience.

The Tauren is liked because of their ability to warstomp (2 second stun with .5 sec cast time, 2 min cooldown)as well as a 5%hp boost.

The Orc (what I play) is liked because they get blood fury for 15 seconds with 2 min cooldown, 15% increased stun resistance and 5 axe expertise. Blood fury boosts attack power & spell damage (The amount increases as you level up higher). If you are planning on specializing in the enhancement talen tree, the orc is generally preffered because of the axe expertise. In the long run, it's all a peference of the player. Either race has its strengths and weaknesses.

I realized that I left out Toll Shammys... You get regeneration and berserk. I haven't read up on them too much, but I know that Tauren and Orc are the current favorites out there.

Trolls make ownage shammy regetion helps a lot in PvP its a trolls +5%  HP thing and Berzek (stuff my spelling) 10% attack speed and if u r below 70 % hp its 30 % attack speed (that + fluuly =ownzs