I chose power. SK can never have enough power. Your healer will provide you with all the extra HP you need. But if you lack power to taunt and hold aggro from healer-guess what will happen 
Also, at higher levels and on raids you won't be tanking much-your HP won't be as crucial as your power. And in groups-well, you will much more often have HP buffs than power buffs. I really see no reason to go extra HP even with my dark elven SK. Raid mobs that hit for 2k+ don't care if you have 100 more HP. They will care if you have 100 more power so you can land one more taunt