So old math says that when it comes to stacking survivability on tanks, protection is always the primary you want, and health mods are better than defense.
However, the amount of damage that characters can deal out these days has increased significantly, so I am going to run some numbers that show how the value of modding for defense has increased.
Now I run K2S0 as my tank, and he does very well for me. Mine is max gear.
His base stats are as follows:
38.5% armor / 399 armor rating
3 health sets would give him 4723.95
3 defense mods would give him an additional 3.31% defense in addition to the 2.39% from the defense diamond all characters get.
Now we have characters like Raid Han that can hit for 30,000 damage twice. With damage reduction before mods, that would drop to around 17832 before the mods. Running defense mods would lower that by an additional 747 damage.
4723.95/747=6.32 If my K2 has enough combined health and protection to live through 7 or more hits, the defense mods are better survivability, if he can survive for 6 or less hits, the health mods are better survivability.
7 * (17832 - 747) = 119595 If I can stack K2's health and protection above that mark, it is more useful to use defense mods than health mods.
Which brings us to my second scenario: protection primaries vs. defense primaries. at 38185 protection, each protection primary stat is worth: 8973.475 protection.
I can add up to 3 defensive primaries to this character as well. Now because defense works on a rating system unlike protection, each one will carry a different value that will also vary based on whether or not I am using defense mods. I will now list the armor percentages each additional defense primary will grant both with and without defense mods. I'll start with using defense mods.
1st: 1.83% or 549 less damage from Han or a hit for 16536
2nd: 1.71% or 513 less damage from Han or a hit for 16023
3rd: 1.6% or 480 less damage from Han or a hit for 15543
with health mods:
1st: 1.98% or 594 less damage from Han or a hit for 17,238
2nd: 1.86% or 558 less damage from Han or a hit for 16680
3rd: 1.73% or 519 less damage from Han or a hit for 16161
WE can see that at no point do defensive primaries provide more survivability than protection primaries. Now this is looking at it in a sandbox. Leads that give a large % defense boost like CLS, Gar Saxon, and General Kenobi may increase that, but it depends on many factors.
Now with my K2, we have just determined that protection primaries on mods are where it's at for him. This also means that Defense mods are where it's at for him since it will make his health pool large enough for them to be better than the health mods.
I'm going to hazard a guess here, and say that this is likely to be true for the majority of max gear tanks now: Defense mods with prot primaries is the best survivability you can get.
Defense conversion was done using this calculator: