My roommate has a Deathknight main with a boxed Resto Druid.
As Unholy he takes a lot less damage (requires fewer heals) and for his play style kills a lot faster.
He tried Blood and found he needed more heals and fights took longer to finish.

I'm not sure if it was because he leveled 55-70 as Unholy so knew the spec very well.
And only did Blood from 70-71 before switching back to Unholy.
I know for both specs he always chose the tanking talents first, dps talents second - and that he used the tanking presence, not the dps presence.
Personally, I'd have gone with dps talents first, and dps presence, because of the back up healer I'm always grouped with.

I've not leveled a DK yet, so do not have personal experience.
But that's my roommate's opinion on their specs.