-HP and -DEF/-RES the same?

If engaging in multiple battles and/or its a slow unit thatll get doubled. -3hp is better since the -3 def results in 6 extra dmg.

For fast units that are only ever supposed to get hit once, either bec of desperation or because theyre only a one trick pony. Then it doesnt matter as long as shes surviving the matchups shes supposed to survive.

Also ur taking looking at phy bulk in a vacum. Ur not taking mag bulk into account.

Ps. Ur simplyfing this WAY too much and ur going in the wrong direction

It's different.
HP normally have less value than defensive stats, with an exception of panic ploy, and stuff like vengeance.

the problem having less defensive stat is the amount of HP you loss.
in your example, let say a healer can only heal for 1 damage.
with your example, -HP will be at full health, which means he can proc stuff like atk/spd push, QR2,guard, or double lion, or anything that have 80% HP treshold.
while -Def one still at 4/5 which means if you only have QR1, he can't double anymore on enemy phase.

also there's cheater like bold fighter armor, or dude named ephraim and loli named myrrh, they can force double onto you, which means your -def one will take 4 damage, leaving him at 1 HP, while -HP one only take 2 damage, leaving him with 2 HP.

so -HP in general is better imo, but -def/res is actually good on some unit like for example Lonqu, you can throw away his def/res, and start using vengeance, since his HP is so thick anyway, he will at least tank 1 hit for you.

-HP is easier on most units because you can typically refine a weapon to make up for it.

Besides that, everyone else explained why.

Adding on to what the others said, it also depends on the unit. For example, I have two Brave Roys, one with a def bane and one with an hp bane. I chose to use the def bane because I could always patch that up with a support skill such fortify or tactics. An hp bane Brave Roy would forever be gimped in comparison.

I think it's best to classify units into three types:
1. Units who will only survive one hit to either defensive stat (ex: most of the mages released in the past 6-ish months, like, Ishtar onwards definitely). When tanking a single hit, HP is king unless you get hit with glimmer/astra. It is mixed bulk, and it has the added benefit of rendering stuff like black luna less effective because those specials scale off your def or res stat. Brave weapons and AoE specials count as two hits for the purpose of this calculation. After taking this hit, it's OHKO your opponent, desperation, some kind of sweep, or die to a counterattack, so focusing bulk in HP and giving up points in the higher of the two defenses instead makes the unit hard to OHKO.
2. Units who will survive multiple hits to either defensive stat (ex: Sheena). -HP is better than -def or -res because if you take -def or -res, every time that stat gets targeted you take 3 (or 4) more damage, whereas if you take -HP, you "take 3 damage" once at the start of the battle and it's done with, over the course of multiple hits you will take a lot less damage.
3. Units who will or may survive multiple hits to one stat but not the other (ex: Bartre, Micaiah): sometimes slightly ambiguous; depends on the unit's spd stat and what you're trying to do with the unit; bane in the lower defensive stat is potentially preferable to make the unit as tanky as possible in the stronger defensive stat. On someone like Cherche or Bartre, who might very well use panic ploy, -res is probably preferred also to keep HP. But on unmerged Ares I might go -def to preserve his ability to take one hit from someone like A.Tiki, getting him into vantage range (and in the case of Tiki specifically, since she's slow, double her for a KO using a special and a naturally high atk stat). I would be giving up to a significant extent his survivability on a second hit to his def (which is otherwise pretty decent), but if I'm really focusing on the vantage OHKOs, getting it rolling is an important part of the strategy and being able to take a second hit to def is not so important if I can get it rolling, so -def is the ambitious try.

The exception is percentage-based reduction (defensive specials, deflect seals, Divine Tyrfing, Crusader's Ward, etc.) which tank better using HP than def or res. See the shield pulse build on Raven/Karla/Lon'qu for how it works. Percentage-based reduction is in play for Micaiah, for example, vs someone like B.Lyn (whom she will OHKO back if brave or firesweep bow are not involved) so -HP is slightly more effective than -def, but the difference is very small.