Gnome's are worthless for fire. There may be bath to support gnomes being better for arcane, particularly later into cata when the 5% scales better... but I haven't looked it up.
For alliance side, I'd say worgen is best (if you can live with the silly casting animations). While in this tier the 1% draenei hit is very valuable, the worgen speed boost cd will find use on many encounters, and once we get closer to t12, the 1% crit will outshine the draenei 1% hit by a large margin.
I haven't done any of the horde side math though, so I can't comment on that.
Play whatever race you want; it's ultimately your decision.