pvp-wise, in alliance, only race worth picking is gnome for any class, as far as racials go;
the other racials are mediocre at best, unless u got some uber-wtfpwning-superhigh-end trinket that will give u an edge as a human.
pve-wise i'd say probably human. gnomes just get 5% more mana. which is good only for discos.
while the human +3% spirit is good for both specs, and even increases your healing if ur holy.
above all these tho, i think people should pick the one they like most, regardless of racials.
judge by looks, or even lore. not by racials. especially if ur not interested in doing arena.
i'd go female draenei, as a healer priest, for both lore and looks ^^