Okay, I was able to play with the Cauldron last night. First off, it doesn't stack for the thief; you get no additional mana regeneration out of health potions. Second of all, it's definitely breakable with the Gnomish Earthmother followers. Once you're ready to fight the boss, use all piety on greenblood then start whacking plants to build up corrosion. You'll hit 100% mana potions before long, so all you need to do is boost your max mana a little and you're in business. Pretty much useless outside of this, though, since the numbers are so low you'll never get appreciable stacks to fuel it without crippling yourself.
Reason I'm not voting is that I don't quite like either item in their current form, and would prefer the devs iterate a bit before we settle on one. If the cauldron does end up being scrapped, I hope the sprite gets reused for a subdungeon item or something, because the idea of an adventurer hauling around a giant iron cauldron as a standard piece of equipment is just way too adorable to pass up.