Percentage life vs vitality?

Some items have percentage life other items have vitality. What is an easy way of figuring out how much vitality the percentage equals and vice versa. Is it good to stack lots of percentage hp?

The basic equation for Character Sheet HP where V=Vitality, L=%Life (ie., 1% Life = 1):

HP = (276 + V*35) * (1 + L/100)

Taking the partial derivatives:

dHP/dV = 35+35/100*L

dHP/dL = 2.76+0.35*V

Now assuming that 200 Vit = 12% Life for most items (Diablo Inc Gamers - Item Affix Quick-look Chart), we have:

dHP/dV * 200/12 = dHP/dL

583 + 5.83*L = 2.76 + 0.35*V

L = (0.35*V-580)/5.83

Which means that given V vitality, you should optimally equip L life:

V       L
1657    0
1800    8.6
1900    14.6
2000    20.6
2100    26.6
2200    32.6

Caveats: Trading 1 stat for another is seldom simple. I used the ratio of the max possible roll on Belt,Chest,Helm,Shoulder (200/16=16.7 vit per %life). Using a comparison of different items will change the numbers (Amulet: 300/16=18.8). Another example might be gem based (34/12=2.83), changing the comparison drastically (I think this is flawed, but YMMV).

Comments: I did this analysis in a rather short period of time, its not unlikely I made a mistake. Corrections welcomed.

The fact that it is a percent means you can't compare it to a raw number, because the value it provides changes based on how much health you already have, and even simply between higher levels (as vitality starts to provide more and more health).

The easiest way to compare the two pieces of gear is by trying them both on and comparing your HP totals while wearing each piece (the change in vitality on the tooltip is misleading since it can't take the % health into account).

As to stacking them, similar to how you shouldn't forego weapon DPS for primary stat, you want a mix of both raw vitality and % health, as having some of both is going to provide more effect than stacking one to the exclusion of the other.