The best shadow priest race's are horde Undead for the devouring plauge spell all dmg done is healing the caster and for Allince Night Elf for the Star Shards spell(free dot) 30 sec CD cost no mana.


I have a 70 NE but went holy and lv'd as shadow 

No, Night Elves are not the best Shadow Priests.  Star Shards is some free extra damage, but it's not a lot of damage and because it's Arcane, it doesn't effect VE or VT.  Draenei are the better choice for Alliance because of Symbol of Hope and Inspiring presence, they may do ever so slightly less damage, but 1% more hit to your part from Inspiring presence is huge.  And, since a Shadow Priest is there primarily to restore mana in the first place, getting that much more mana back from Symbol of Hope is better.

IOW, the extra damage that the hit gives you (because your party members can trade out some hit gear for more crit and/or damage) along with the extra mana, it completely trumps Star Shards.