If you guys really want to know for sure - make characters on the PTRs.
I spent over 30 hours testing shammys and mages in 5 and 10 boxing configurations.
Shammys in theory are so much better.
In practice and in real PvP (I did not test PvE) - the mages were FAR superior in both shortness of downtime, ability to insta gib, ability to instant shield and heal and increased mobility (blink, invis).
Granted, I have 2 years of playing mages under my belt and I am already a bit biased towards mages anyway but honestly.... the shammys were not anywhere near as good as I would have imagined. Even with some upgrades of macros and more practice.... I still see them as weaker than mages/priest in large scale PvP. In arenas and PvE situations - that may very well be a different story.