I was recently asked to write a post about tank stat priority for Powertechs. Unfortunately, it turns out to be pretty difficult to just make a list saying "this stat is twice as good as that stat". This was possible on WoW due to exhaustive simming and combat log analysis, neither of which we really have for SWTOR yet. Also, since absorption and shield chance depend on each other, it is clear that the relative value of these stats will shift as the amount of the others changes. Here are our main defensive stats as tanks:
Endurance: This determines health, giving about 10 hit points for each point. There isn't any real way to gear for endurance besides augment slots, which I do not advise. Stacking health is mostly only useful when you are being insta-gibbed.
Armor: Armor provides the bulk of your energy and kinetic damage reduction. Like endurance, you pretty much just get the amount that comes on your gear.With armor comes damage reduction in general. All of us get 5% from Ion Gas Cylinder. Optionally, there is also 2% from Ion Screen in ST, 2% from Power Armor in AP, and 4% from Combust in ST (never displayed on the character sheet since it is a debuff).
Defense: Determines chance to completely avoid an attack. 5% base, 2% from Infrared Sensors in the PT tree.
Shield: Determines chance to shield part of the damage of an attack. 5% base, 15% in Ion Gas Cylinder, 2% from Shield Vents in ST, 10% from Empowered Tech in ST.
Absorption: Determines amount of damage reduced by a successful shield. 20% base, 6% from Ablative Upgrades in ST.
SWTOR has a two roll system for attacks. First, the attacker (the boss) rolls to hit. We assume the boss has 100% accuracy. The boss is rolling against your defense chance here. If he successfully beats your avoidance and lands a hit, it goes on to a second roll. This determines whether the attack is a crit, a shield, or neither. If crit and shield chances add up to over 100%, crit starts reducing shield chance. This is not something we reasonably have to worry about.
Gearing for us will probably be a constant battle between defense, shield, and absorb, with the relative values of each shifting as we gain new gear and trade out mods. The stat we want the most constantly changes as we aquire more of the others. While defense may be the worst stat for you to gear for today, it may be your best tomorrow after a new chest or legs. The goal is always to find a balance between the three that gives you the greatest average damage mitigation.