672 armor is higher in value for effective health than 6 stamina for any physical damage based encounter. Magic damage based encounters the 6 stamina would naturally be better as armor has no effect on magic. The avoidance doesn't really enter into the equation much into it in ICC or Ruby Sanctum as both have avoidance reduction.
The one from Rotface 25N is better for tanking physical damage.
The one from Lord Marrowgar 10H is better for tanking magic damage.
You aren't very likely to pick up Lord Marrowgar 10H one for the additional 6 stamina just for tanking magic based encounters. More likely you would probably pass it to another tank first and probably pick it up later if it is going to be sharded. Least that would be my thought.
"In anything, if you want to go from just a beginner to a pro, you need a montage." /w TankSpot WTB Montage for Raiders.