You can't make a blanket statement like "HP does more than MR." It's all relative. If you have 10,000 HP and 0MR is 500 health or 50 MR going to do more for you? (Spoiler it's 50MR)
Here's an example of a midlaner who got one of the higher values of HP compensation buffs: Anivia.
She went from 467.6 base HP to 480, and her growth stat went from 70 to 82. Anivia has 30 base MR.
So at level 1 Anivia used to have
467.6 HP and 42 MR (30 base + 12 from MR runes). 100/142 = 70.4% attack modifier (a 29.6% damage reduction, or any damage dealt is multiplied by .704. E.G. 100 magic damage would become 70.4)
If we take 467.6 / .704 we get 664.2, or Anivia's effective health pool vs magic damage....that is, the total pre mitigation magic damage she can take before death. This is evidenced by taking 664.2 x .704 we get 467.6, Anivia's max health at level 1.
After compensation buffs, Anivia has
480 HP and 30 MR. 100/130 = 76.9% attack modifier
480 / .769 = 624.187, or 40 EHP less than before.
With the increased 12HP/level growth stat, it's not until level 7 that Anivia's EHP is actually higher than before runes reforged.
Level 6 =
467.6 + (70 x [level - 1] or 5) = 817.6 [level 6 HP pre compensation] / (100/144 or .694) = 1178.097
480 + (82 x 5) = 890 [level 6 HP after compensation] / (100/132 or .758) = 1174.142
Level 7 =
467.6 + (70 x 6) = 887.6 / (100/144.4 or .693) = 1280.8080
480 + (82 x 6) = 972 / (100/132.4 or .755) = 1287
So, as you can see here, you are wrong, at least for the early game where these small number changes are significant. Many other champions who got the compensation buffs are even worse off, and some got nothing at all.