Basic Calculations

Say you are running a 30% HP Lead, and have 50% mitigation. You should have 2.6x EHP.

At this point, a 1000 HP base unit can take 2600 damage before dying.

With a Legwand, you can have 3.1x EHP.

At this point, a 1000 HP base unit can take 3100 damage before dying.

Now, if you drop the Malice, and use, say, Spirit Tiara. Spirit Tiara recovers 10~20% damage when hit (averages to 15%). Spirit Tiara gives you a 1.176x multiplier on EHP that stacks multiplicatively. The math comes out to 3.06x EHP

At this point, a 1000 HP base unit can take 3060 damage before dying.

Spirit Tiara is already close, and that's not counting the added DEF or BC Fill on hit. The main disadvantage is that you can get nuked 100-0 and die before you heal, but in most content you'll have some time to heal between hits. You are also a bit more susceptible to RNG.

More Math

Tridon 30% HP Mitigation Spirit Tiara = 6.67x EHP

Tridon 30% HP Mitigation Malice = 6.67x EHP

Now, you might ask, why wouldn't use a Occult Treasure? Wouldn't it give more HP?

Well, yes. But the point is that Recover HP when Hit is way stronger than it looks, especially when you stack it in the secondary sphere slot/ES, and I think that it should be considered more in teambuilding/sphere selection. In situations like raids, it is invaluable in keeping weaker units alive.

TL;DR Spirit Tiara offers roughly the same EHP as standard stat spheres. This is ignoring the ATK/DEF/REC that Spirit Tiara gives. Using a Recover HP when hit sphere in your second slot is usually going to the best way to increase survivability.