I think the value of each shield depends on it's use like Dragaan is suggesting. I'm not too skilled with math, but some of the math that I can do will hopefully help out with the decision of which shield is better at a particular application, or just better overall.
Some things to take into consideration:
-These calculations are applied to your current gearing setup.
-For the effective health calculations I used the stats from from your Armory profile with shield enchant (defined as +18 stam) and without it, and shield specific gemming with and without gems (defined as +30 stam, and 15 stam +10 dodge rating).
-I didn't count the EH increase you'd get from the black heart trinket, but I did count the +126 stamina.
-I didn't calculate the EH for the shields specifically
-For the actual EH calculation I used the calculator provided on Effective Health Calculator: How much damage can I can take? - TankingTips.com.
-Avoidence percentages are before diminishing returns.
-Threat ranking will be based off of modified BV. You're a warrior so it'll impact your threat production.
-I didn't take into account diminshing returns for Shield Slam.
-For other calculations I used the calculator on A list of useful Tanking Formulas for Warriors - Dodge, Parry, Block
-Different gems, enchanting, and procs will modifiy the values, so I left this out to keep it alittle bit more simple for myself, yes I'm lazy. Yes it isn't a complete picture, but I think it's enough to make a decision about which shield is better than the other.
-Also, someone please verify my numbers, my goal is to help make an educated decision about which shield is better. = )
==============Boreal guard ====== Crystal Plated Vanguard =====
Effective health: 90,438 vs. 93,960
EH modified: 94,393 vs. 94,423
Avoidence: 1.72% vs. 1.7%
Avoidence modified: 2.07% vs. 1.7%
Threat: 242.5 vs. 384.5
Conclusions: Effective healthwise, prechanting and gemming the Crystal Vanguard comes out ahead. If you enchant both shields with +18 stam, and gem the Boreal Guard with your current gemming setup (+30 stam, +15 stam +10 dodge rating) the Crystal Vanguard still comes out ahead although slightly (+30 EH).
For avoidence the Boreal Guard beats out the Crystal Vanguard prechanting/gemming slightly (0.02%) and even more so with your current gemming and chanting plus the socket bonus (0.37%).
Threat production should be easier on you with the Crystal Vanguard due to the fact that you'll be dealing more damage over all because you have more strength with it than you do with the Boreal Guard (+32 str). Also with your current spec, when you get hit by a mob/boss you'll deal more damage with damage shield from the Vanguard than you would with the Boreal Guard (76.9 vs. 48.5). Your shield slams prior to diminshing returns should hit harder with the Vanguard because of your higher BV.
If you're going for threat and effective health go with the Vanguard, but if you're leaning towards the avoidance side go with the Boreal Guard.
I didn't mention this in my post but the parry and dodge rating calculations on A list of useful Tanking Formulas for Warriors - Dodge, Parry, Block isn't updated for patch 3.2
As of patch 3.2 a dodge rating of 45 = 1% dodge chance, and a parry rating of 45 equals a parry chance of 1% before diminishing returns. So in order to calculate the percentages I Followed this formula:
Dodge OR parry rating/45 = avoidance % before diminishing returns