HP is a stronger general defense.
Also armor/mr makes you 1% harder to kill per point. So 2 armor or 2 MR is the same as having 2% more HP facing those damages. If someone needs 3k damage to kill you, now they need 3060. Where as with 3% more Hp they'd need 3090 no matter their damage type. Including true damage.
And this becomes less effective as you have more armor/mr.
Say you have 50 armor and 2k hp. This brings your effective Hp to 3k. Adding 2 more armor brings this to 3040. Even less EH.
Now if you add in 3% Hp. Now you're at 2060Hp. Times 1.5(50 armor) and you have the 3090 again.
Not only is it less effective HP, it only guards vs one of the three damage types.
Only time it may shine is when shields and heals are involved. Where armor/mr makes them worth more per HP point. Even then it's iffy and still only vs one type of damage.