It kind of depends sometimes. Many groups would like a power bard to keep them filled up. When you get every power spell, you can pretty much become a battery.
Then again, a lot of tough mobs in the game have an AoE crit, so you could use the blood to heal the tank/melee that were affected.
I personally would like either one. Most of the groups I've been in have been a couple levels higher than me, and sometimes my heals cant keep up with the tank, so the Necro could either blood the mob so he can heal the tank and/or anyone else that mightv'e been hit, or use a power spell to raise everyone's power so I can use another heal/stack heal.