Between the two choice to stack on your 6 armor, what would you choose for a D/D condition thief. Although I increase condition dmg, I also like to backstab/auto attack. So my build is pretty much geared towards precision + condition dmg, with a bit of toughness/vit here and there.
Crest of Rabid after equipping all 6:
+120 Condition dmg
+ 84 Precision
+ 84 Toughness
-Roughly 12s total
Superior Rune of the Thief after all 6:
+165 Precision
+57 Condition dmg
+10% dmg increase when attacking from side/behind.
-Roughly 8g total