+HP is useful across the board, since heals and natural uptick are done as PERCENTAGES of the HP (higher total means that the same % is a higher amount of points recovered). Not only that, but it applies regardless of what you're being hit with. Regen is also better to slot than resistance (usually) for the same reasons.

Regeneration is based off of current maximum health meaning +MaxHealth increases regeneration in total points. But heals are based off of original base health: buffing +MaxHealth does not increase heals in total points per heal. The old Dull Pain guides did the math and the testing to confirm this. Slotted Dull Pain did something like 78% heal but visually it looked closer to 50% heal because that was 78% of the base health bar, not the boosted one. If you have 1000 points of health and apply a 50% heal, that would be 500 heal. If you boosted your health to 1200 points, that same heal would heal for the same 500 points, or visually something closer to 40%.

The reason for this is because actually heals didn't heal percentages at all: they actually healed based on a table calculation similar to how attacks dealt damage. Those tables were calculated per archetype to make heals come out to nice percentages (most of the time) but they did not change when players buffed MaxHealth, so there was no way for a MaxHealth buff to make heals scale up.

The question "all things being equal what's better, +MaxHealth or +Res" is a tricky question because the premise - all things being equal - is never, ever true in CoH. It can't even easily be engineered to be true on paper in an academic question, because of the question of what does it mean to be "all things being equal." All we can really do is compare incremental benefit, which means given a specific set of circumstances, we can ask whether +X MaxHealth buff is better, equal, or worse than +Y Res. Outside of those specific circumstances, any comparison is likely to be conditional. Sometimes this is better, sometimes that is better.

Because players sometimes thought about things in terms of points, and sometimes in terms of percentages, and game effects sometimes conveniently worked in ways that appeared to be simple in points and sometimes in percentages comparing effects can be difficult. To "normalize" the comparisons, I came up with a system for directly comparing a specific amount of resistance and a specific amount of +MaxHealth by converting the +MaxHealth buff into an effective resistance. Basically, it works like this (warning: math inbound). Take your total MaxHealth buff. Say its +40%. Calculate this number: 1 - (1/ (1+buff) )). In other words, if the buff is +40%, then first you calculate 1/(1+0.4) = 1/1.4 = 0.714. That's actually the apparent damage admittance: damage will appear to be hitting only 71.4% as hard. Then you calculate 1- that value, which is 0.286. That's the effective resistance: the amount of reduction in the damage.

So +40% max health "looks like" +28.6% resistance. Note that you can't add or subtract these numbers: you cannot "stack" any true resistance on that apparent resistance value: that requires more complicated math. Now, there's one big catch to this. In this comparison system, you have to assume that your effective resistance value - in this case 28.6% - is not only your resistance to damage but also

*your resistance to heals*. In this system, all of your self-heals will heal for 28.6% less. This is due to the fact that heals do not scale. They heal for the same amount with or without +MaxHealth, but in this new system where we are making +MaxHealth look like +Res, heals are stronger for +Res for mathematical reasons.

In a perfect idealized world without debuffs and the like, +40% Max Health looks exactly like +28.6% resistance in all respects except it will seem like the maxhealth character is getting exactly 28.6% less benefit from heals. However, the max health character has that effective damage mitigation against all forms of damage, even unresistable damage. If I took all Real Numbers and combat spam away, and the only thing you could see was your health bar, outside of heals, if the only thing that happened was something attacked you with damage of the resisted type, you couldn't tell these two situations apart.