(I hope it's okay if I double-post here. It's been a while since my last post too, so an edit would make little sense.)
I've done some calculations on the whole armor vs health thing, and I think that bonus health isn't all that useless actually.
First, you have to think about what is important here. What's the goal of a tank pet? I'd say it's surviving as much damage as possible. +5% health means the pet can survive a whole 5% more damage. +13% armor increases armor rating, and we all know the more armor rating the pet already has the less of an effect extra armor is going to have.
Let's do some maths. A lvl18 tallstrider (+0% armor and +5% health) without any pet talents has 1028 armor, and 503 health. That armor reduces the damage from same-level mobs by 1028/(1028 + 400 + 85*18) = 34.75%. If we combine that with its health, the pet can tank about 770 physical damage at full health. A turtle (1162 armor and 479 health) can tank only 767 physical damage at full health. The difference is so small that we can safely say the two pets can tank physical damage equally well.
What about magic damage? This is where health really shines. Armor only reduces physical damage, not magical. Therefore only health affects how much magical damage a pet can tank. For our lvl18 tallstrider that's 503 damage, and 479 for our turtle. The difference is a little bigger here.
Now, what about if we take Mend Pet into the equation? Shouldn't the turtle's stats really shine there?
For a pet with +0% armor, each HP is worth 1.187 damage. For a +13% armor pet that number is 1.211. This means one secnd of Mend Pet Rank 1 (20 HP healed) heals 23.74 damage for a +0% armor pet, and 24.22 damage for a +13% armor pet. That's a very small difference. It's still a difference, but considering that Mend Pet mid-fight is only useful if you can heal faster than your pet is dying, it is probably rarely relevant.
I'm not trying to say Turtles aren't superior tanks. They are, but not because of their stats. Bears are the best tanks just looking at stats actually. If we do the same calculations I did for the lvl 18 turtle and tallstrider a bear (+5% armor, +8% health) can tank 806 physical, or 517 magical damage. That's more than either of our examples above.
The reason why a turtle is still the best choice if you want the tankest tank you can get is shell shield. No matter how much +% health or armor a pet is going to have, it will never be as tanky as a turtle with shell shield on. It reduces ALL incoming damage by 50%, that includes magical. That's only for 12 seconds, but still.
My conclusion to this is that I was probably right to think tallstriders were nice tank-that-can-still-DPS pets. Not as DPS as a cat, or as tank as a bear/boar/turtle with shell shield, but enough of both that you can take it in either direction with the right pet talents. There's also a tallstrider with 1.3s attack speed for PvP, that's something Turtles don't have (neither do boars and bears). As my second stable slot I'll get a turtle, in case I ever need something tankier than my tallstrider.
EDIT: Turns out I still had Endurance Training and Thick Hide in my talents when I checked those values. The pets however did not have any talents, so the difference is probably irrelevant.
EDIT2: Just to be clear, I am not saying +5% HP is strictly superior to +13% armor. All I am saying is that - despite looking like a much smaller bonus (5 < 13) - the +5% HP bonus comes surprisingly close to the +13% armor bonus, making it just as viable as the armor bonus if not more. At higher levels (and with the Natural Armor pet talent) the armor bonus is still going to make your pet tank physical damage more effectively than a health pet, especially if you take healing into account. However the health bonus also helps against magic damage, making health pets more versatile tanks than armor pets.
