Hp is good, but I prefer ac, if playing a class that can easily and quickly heal themselves, as others have said, it does more then just damage reduction. I watched Suunmar, take hits/injures for damage. 10 rounds, say fighting a warrior dwielding that hits all 8 attacks, doing that sort of damage is taking what, around 10-15 hp per strike, 80-120 damage per round. That same warrior hitting you for mauls/devestates, is dropping, prolly 20-30 hp per strike, so 160-240 damage per round, which my math is prolly off on what noun does what range of damage, but thats just how I see it. 10 rounds first example (and I rarely see warriors dwielding whips hit -all- eight attacks, every round) they take 800-1200 hp from you. Second example, they've taken 1600-2400 hp in the same amount of rounds.
I'd find a tough mob, that hits hard, and fight them, vlad for example. If vlad is doing caps damage to me consistently, as a communer, I go for more ac. If I'm taking less then caps damage, I focus on plus mana gear, saves, hit/dam roll as needed, unless I'm playing a cleric, then I don't really focus on saves as much, just because of spell turning, only really have to worry about aoe spells, those are mostly afflictive in nature, so kinda narrows what sort of saves to go for.
Edit: It takes me forever to post tonight, what foxx said
edit 2: And pali
Edit 3: If you're a cleric/paladin/healer, cure crit is what, 20ish mana? If I have an item thats plus 20 mana, or plus 20 hp, I take the mana, because cure crit does translate to about 50 extra hp. I'd rather start with 20 less hp, and be able to heal 50 worth of damage.