For classes that can cure themselves, AC over HP.
For classes that cannot (with the exception of battlemage), HP.
I agree wholeheartedly with this, in theory.
I'll give you an example (that is 100% not true to game mechanics at all by the way, it's just to give you an idea of what I mean).
Cleric 1 has 500hp.
Cleric 2 has 1000hp.
Cleric 1 has more AC than Cleric 2, enough so that Cleric 2 over time takes twice as much damage as Cleric 1 (remember folks, this isn't a real scenario).
So when Cleric 1 and Cleric 2 (who assuming all things equal, will die at the same time if they're fighting the same opponent with things going exactly the same in both fights) are both at 10% of their max HP, which one needs less time and mana to be at their full health and effectiveness? Cleric 1, with the less HP but more AC.
Now again, it doesn't work like that at ALL in-game, and you can't disregard the benefits of HP gear with one of the low-HP cleric races at all. But fantastic AC, in theory, really is a good thing for healing classes. I say in theory because I've never tried and tested this myself so I can't throw in my own experiences, so remember: THE ABOVE EXAMPLE IS NOT TRUE TO FL LIFE, IT MERELY SERVES AS AN EXAMPLE! 
Like Calron says though, it also depends on what you're sacrificing for what. Sacrificing that +40hp breastplate for one that gives you an extra -5 AC isn't really worth it.
Dey