I've been wondering the same thing. For example, let's say 2 different items are up on the GTN for roughly the same price. One will increase my primary stat by 2 and add 20 armor, but the other will increase my primary stat by 4 with no change in armor. Which one is better?

If it helps any, I just found these stats on another forum:

1 Endurance = 10 HP

5 Endurance = 1% HP regen

20.8 Armour = 1% damage reduction

Here's a quote from SWTOR's combat designer about how combat works, which I found

here:

"We use a two-roll system for determining combat results.

First is a hit roll, accuracy versus defense, and if the attacker misses then no damage occurs. If the attacker rolled poorly enough to miss even discounting the target's defense then a "Miss" result occurs. If he misses because of the defense then the result varies based on the attack type, the cover state of the target, and the target's equipped weapons. All the possible results - Dodge, Parry, Deflect, Resist, Cover - are mathematically the same, but they can trigger different effects and are visualized in different ways.

If the attacker hits, then a second roll is made with the crit chance of the attacker versus the shield chance of the target. If a Crit or a Shield occurs then the damage is adjusted up or down (based on Surge/Absorb), and then it goes through to the armor and damage resistance. A critical can never be shielded, and an attacker with a high enough crit chance can push the target's shield chance off the table. It shouldn't be possible to get your passive crit chance high enough to start pushing off the target's shield chance, but there are short-duration buffs that push these chances high enough to come into conflict."

So, as far as I can tell, things like shielding & armor are always tertiary to your primary stat & Endurance. Your primary stats determine whether it's a hit or not, then there's a crit vs shield roll, and THEN armor and damage resistance come into play. Armor is basically a third-order effect, so the other stats appear to always be way more important.