wowhead's compare tool doesn't like my work laptop so I have to eyeball this.
But, you gain:
8 DR: (about 0.6 actual defense)
5 dodge rating (0.13% dodge before diminishing returns)
38 hit rating (1.16%)
translation: I cannot see any possible reason to not use the gargoyles. Your worst fear here is MAYBE going under 540 def. If you were just squeaking by 540 right now you might. But for most tanks at this level it's unlikely. The dodge loss after DR is so tiny it's barely even there. And the hit rating...well HR is always nice, but I'll take 600+ armor over a little hit any day. Esp since Bladed Barrier/Armed to the Teeth are going to give you more AP, and so is the increased strength. That should make up for any possible loss of threat.