In general I feel there is too much emphasis on "averages" in this forum. There is no doubt that war hero relics will provide a higher average mitigation in just about any fight you can imagine. The point I want to make is that there are often periods in every fight where the incoming damage is very high above average. This is the point in time when defensive stats are at their highest potency. Deflection is 5% melee/ranged defense on average. Pretty awful - but if you use it at the right time you become nearly immortal. There can be no proof that one is better in general than the other because it depends on the fight. I will leave it to the individual to decide which is better. This is how it should be, because when new fights are released there is no one to ask which is better and every fight can be defeated with different strategies.
Averages reflect the vast bulk of game content. Boss fights are a comparable minority, and a smaller minority yet are ones that severely stress a tank's survival and healer resources. They're out there, but from a time perspective, something like 95%+ of one's interaction with the game falls into encounters where the statistical average is applicable and useful.
Luckily, relics are sufficiently easy to obtain that picking up a variety beyond what is best-suited to the average case (e.g. ones that are quite obviously well-suited to a particular tank during spike-damage phases) is not excessively onerous.
At worst, one totes around an extra piece of gear per relic slot to accommodate swaps, which is not, to my mind, excessively onerous either. For myself, as an Assassin, I do alternate between a static Defense relic and on-use Shield/Absorb relic, and it requires only a single swap (at present my body armor, once fully re-optimized for available 63 mods on my server, an implant) to keep overall defensive stats in line with the mathematically optimal numbers.
Obviously, there are specific fights which hugely benefit from intelligent timing of on-use relics and normal defensive CDs. That will always continue to be the case, and good tanks will always carry additional relics to be prepared for them. When new content comes out and we don't already have a model for the fight, the first couple attempts will undoubtedly be trickier, but it doesn't take long to figure out whether any given encounter mirrors the statistical average or has properties that make it an outlier and thus better suited to a slightly changed gear makeup.
And really, this entire discussion boils down to relics, because there is precious reason otherwise not to have a single tanking gearset optimized for one's particular class (there's an easy-to-calculate mathematical "best", from which any deviation statistically means more damage taken). One is never better served by having more Defense for one particular fight, and more Shield for another. Other than ensuring sufficient HP (trivial at present), the math doesn't support a lot of customization.
To bring this ramble back to the task at hand... for the vast majority of overall game content at present, one is best served, mathematically, by EWH Defense relics for Juggy/Guardian. For those particular boss fights with highly non-linear damage profiles, one can easily adjust. The number of tanks operating at a level to be concerned about such things is quite small, and a dramatically higher percentage are critical thinkers able to make sane adjustments without too much outside help (progression raiding, especially progression tanking, tends to attract a certain type of person at higher levels of play, and those people are typically more autonomous). It makes the most sense to recommend the most common "best case" to the masses, recognize that there are outliers, and also recognize that most of the people for whom such things even matter are better prepared to see the limitations of the general suggestions.