I feel like the situation that would call for Andromeda's HP weapon would basically require....
1. You're looking to use your soul for defensive purposes (this is the point where your choice of soul comes down to Joan or Andromeda)
2. You have enough sources of resists/damage cuts from hime such that healing is now a higher need than damage cut (now this is where you choose the Andromeda line over the Joan line)
3. You don't need Maiden's Prayer or Chaos Magic (thus leaving Andromeda as the default)
4. Because in general we prefer +element over +HP for the soul weapon, the skill buff it grants must be much better for the HP to be worth picking, right? So this means that we would need the improvement to the def buff from the HP weapon to be better enough over the addition of regen to the heal from the element weapon to compensate for +element's advantage over +HP. We'll each have our own different valuations here, but to give mine as an example: If I have at least two lv 20 ascension (S) skills in my grid (+48%; close enough to 'plus a half'), I'd pick the +element weapon here. Otherwise I'd go with the def buff improvement from the HP weapon.

Now a +40% def buff isn't bad, per se. For a character starting at 100% defense, getting raised to 140% defense means incoming damage goes down by ~28.5%, when everything else remains the same. Stacking isn't so great though; since it's damage / [1 * (sum of def buffs)]. Which means that the more def one already has, the less impact adding even more does, and that def alone takes extreme amounts to reduce damage to small amounts (which is why Athena's def buff needs to multiply her def by 10).

But again, as I said in the previous post, damage distribution is usually not even. When you really want to mitigate damage, go with the damage cut. They reduce damage more and stack much better.