I run pure Defense/Immortal in PvP and here's how I gear for it. Endurance>>Shield>Absorb>>>Defense Augment for endurance as well. As others have stated, endurance is the primary stat for PvP tanking, not mitigation as a large portion of incoming damage can't be mitigated anyway. As for absorb/defense distribution, I only take 3 or 4 pieces of defense to get right around 11%. This is really just to proc riposte/retaliation more reliably. As an aside, I am currently working on a B mod, heavy endurance enhancement, power/alacrity set with full endurance augments. Results are still out but I can say that the early returns are very promising!