It's easy to calculate a rough equivalence between a stamina trinket and a block value trinket for a given fight assuming you know the average amount a mob will hit you for:
(Your HP / average hit) x 1.1 (
[Eternal Earthstorm Diamond]) x 1.3 (Shield Spec) x BV gained from trinket
gives you the effective health saved, which you can then use for comparison purposes (obviously take out coefficients as necessary if don't have the meta/shield spec).
So for example, if you have 18k raid buffed and the average incoming hit is 2k, the
[Gnomeregan Auto-Blocker 600] will give you 9 x 1.43 x 59 = 759 health (discounting the use effect) or 9 x 1.43 x 92 = 1184 health (averaging out the additional block value from the use effect). The averaged out figure is a bit deceptive (given that the mitigation it provides is only present for 20 / 120s) so I'd stick with the discounted value myself.
In comparison the
[Commendation of Kael'thas] gives 1.06 (Sacred Duty) * 1.1 (Combat Expertise) * 1.1 (Kings) * 57 * 10 = 731 health (and a passive effect which is not accounted for here, but which obviously makes the Commendation a better choice overall).
Note additionally that BV is less valuable if HS drops off (crushings) or you are taking magic damage, whereas stamina is always effective.
For most boss fights you will of course get hit for a lot more than 2k on average, so stamina trinkets will generally be the better choice.