Either 5/5 Seals of the Pure or 5/5 Divine Strength would give the best initial bang for the buck, but I'd tend to go for 5/5 Benediction, which is kind of meh, but it's on the shortest path to Seal of Command, and going for either of the others would mean that you get SoC at level 26 instead of level 21, which is bigger than I had first thought.
So unless you're committed to leveling 10-80 as Prot, start as Ret at least for now, which means chase the Ret tree for a while.