Willpower vs Invulnerability

Ok, just so I'm understanding the OP...

You are generalizing Invuln and WP effectiveness
regardless of whether they are a primary or secondary
power-set?

I have played the game since launch.
Took a break in 2009 until about 2 months ago.

My experience:
Invuln/SS > SS/Invuln starting from base

Assuming consistency (since I have not yet built a full WP build):
WP/SS > SS/WP starting from base

because your primary set gets dominance over your secondary set
and stats are adjusted accordingly

I built two nearly identical invuln builds (nearly because primary and secondary
sets are not identical pools)

One Invuln/SS Tanker the other SS/Invuln Brute, at lvl 24 with exact same
enh's maxed 3-4 slotted (where effective) and at x3 to level per slot SO's
which one do you think was tougher and could withstand normal random +2x2
mobs?

Running the same missions (I admit to using the DFB to PL each set to lvl 24 so I could
test each within a reasonable [before I got bored with the experiment] amount of time) my invuln Tanker not once face planted, while my Brute was becoming expert in floor inspection.

So back to my original question, how can you generalize comparisons overall for each build by using builds where the power-sets in the comparison are being used as secondaries and say that these figures apply to both brutes and tanks as a whole?

My INV/SS Tanker has the following stats.

Without my ATO proc stacked which adds 20% to every resist she has:

90% vs. Smashing and Lethal
33.7% vs. Energy
36.8% vs Negative
36.2% vs. Fire and Cold
34.9 vs. Toxic
32.9 vs. Psionic (yes that's right psionic)

Her defenses without Invincibility saturated (just 1 mob for these values):

36.5% vs Smashing, Lethal, Energy and Negative
38% vs. Fire and Cold
37% vs. Psionic (yep again psionic)

Everything but the psi defense grows with mob size.

Her regen? Well with Dull Pain up (which is perma) she regens 43.6 hp per second. Now that might be much lower than a WP tanker but she's mitigating so much incoming damage her health bar never moves except on the hardest difficulties vs. the highest number of foes that the difficulty slider will allow. Barring extreme -res debuffs (which my ATO proc gives me a decent buffer) and considering my defense debuff protection, I am hard pressed to find mobs that can give me cause to worry.

I'm not sure I could say the same on a WP tanker in the same circumstances.

Some different priorities come up with incarnate powers between the two as well.

My INV took rebirth for her destiny which adds more regen
As a WP I'd take barrier for more mitigation

My INV I plan to take assault for her hybrid...
As a WP I'd take melee +resist

I can't imagine it being much different with a brute (obviously all the survival stats will be lower but thats just reality for the higher damage they do).