That's a decent example, but that's not quite how effective health works. (get ready for a bit of math)
Effective health (for a certain damage type) is created by multiplying your health by (1+(resistance/100))
So, for example, if you have 1000 health and 50 armor, then your effective health for physical damage is 1500. Your enemy needs to deal 1500 damage (before resistances) to kill you.
You can then figure out what percentage of damage you resist by dividing your base health by your effective health, then subtracting from 1
ex. 1000 base divided by 1500 effective, means you take 67% of damage, which means you resist 33% of damage.
I'll throw in another example to show why you need a combo of health and resistances, not just one or the other.
Say you currently have 1000 health and 300 armor (4000 effective). You need to choose between getting 1000 more health or 100 more armor (assume same price)
If you get the health, you will have 8000 effective health. If you get the armor, you will have 5000 effective health.
Now, let's try if you have 4000 health and 0 armor (4000 effective). Same choice
If you get the health, you'll have 5000 effective health. If you get the armor, you'll have 8000 effective health.
As you can see, the better purchase depends on your current stats, and you need a mix of both resistances and health to survive.
Granted, this gets more complicated with resistance shreds, lethality and magic pen, percent damage reduction, regen, true damage, grievous wounds, heals, itemizing against both magic and physical, etc. but you can still see that you need a mix of resistances and health to be an effective tank.