I don't believe Unholy Presence (http://www.wowhead.com/spell=48265) increases mounted speed.
The spell effects, show three effects with the movement portion specifically saying Run Speed.
Or if it does, then Pursuit of Justice is calculated in such a way that a Paladin not using Crusader Aura is consistently faster than 0/2 On a Pale Horse + Unholy Presence.

The second question (B), is more a convenience vs effect question.
Scent of Blood is absolutely mechanically stronger in every way.
But similar movement speed is nice in battlegrounds, which is 98% of my PvP experience.

I'm more interested in the first question.

Which is stronger... 2 additional attack power per 180 armor or 2% damage reduction when both Death Runes are on cooldown.
It seems that Howling Blast uses Frost Runes before Death Runes, Necrotic Strike uses Unholy Runes before Death Runes and Obliterate also uses fixed runes before Death Runes... so the 4/6% reduction is up less than I would think it should be.