When I compare an item in my bags vs the item I have equipped (Shift + Hover over item), the Item Comparison tooltip comes up showing all stat differences between the two items. That is all stats except Armor value.
I don’t know if this is an oversight or a deliberate decision, but I currently have to use another addon (ExtraTooltip), which duplicates the entire default UI comparison, but also includes armor differences.
As a tank, Armor is a pretty significant stat and should be in the default comparison IMO.
If you need to have the system tell you that 2089 is 198 more than 1891 and you dont just can see that its kind of close to 200 more by a glance when 200 extra armor means something like 0.01 more dmg mitigated then im sorry for you but you dont know what stats to actually value more, the important stats in the comparison are for a tank stam, the mitigation stats, hit and expertise armor is something you will have enough of no matter what if you gear in even a semi decent way.
Hopefully someday people will understand than a tank is not only an hp pool but also an avoidance and Armor so they don’t deplete the healer’s manapoolbor, or become basically unhealable by some less focused healer’s class
(I once had to do a full stamina re-gem and put on PvP stuff to take my bear in ToC, it went to hell mid raid but that wasn’t my problem anymore, the healing druid said I didn’t cap defense, as a druid I was disappointed. All that to say that you shouldn’t trust others to know what you need)