Napkin math suggest that for fights in which you can sustain SBlock 100% when the boss is on you, the relic falls back a bit, but it's better than the Shado-Pan/jade trinket. For me the relic gives ~9% dodge (12 sec), the shado-pan ~13% crit block (15 sec). If I had to choose between both I would pick relic until gear goes up. The relic is fairly good with the low avoidance gear most of us are sporting. None of them have an assured effect (which I never liked as tanking trinkets), so pick your poison.
Personally, I'm using both, and using elixirs instead of flasks. I don't think I will stop using the Relic on this tier. Elegon/Tsulong trinkets are good for "a boss is on you 100% of time" scenarios, which favor the relic over the shado-pan/jade trinket, so....