Spike Armor is for melee range, which you'll be avoiding, so it's the least effective of the three skills. Obsidian Shield is a great group tool for the large shield on you with a decent amount on your teammates. However, it's morphs don't serve ranged DPS very well. Green Dragon Blood is an okay heal, but not great IMO (though my only experience with the spell was on a light armor mage). The real benefit for you would be the stamina regeneration for bow abilities.

I would choose Obsidian Shield if you group with other players a lot. Otherwise, GDB is the best skill for you.