Here's a series of tests I did today after some finidings I did earlier this week regarding Gab and CS defences.
Basically I just got attaked with differnt gear on and varying skill with r-hits.
The weapon used was a Raptor+6 with NO bonus, so no poison dmg. (167 AP)
The shields used were Chitin Shield+5 and Gab's Blessing+1 rebirth.
The attacks used were normal r-hits and Howling Sword: Inflict 300% damage with no chance of failure and 200 additional damage.
These test were done with the shields as the only variable per test and additional not mentioned dmg-reducing skills/items (pathos gloves/absoluteness/matchness) were not used.
These were the results:
As you can see the in the results Gab only came out better in the r-hit test. This is because the extra AC from the Gab makes up for the weapon defence loss.
However, as soon as a skill is being used with additional dmg (like the warrior skills and BP skills) the AC does not reduce the additional damage at all. The weapon defence of the CS does however.
We also see that the effect of the CS is strengthened with the more weapon defence used (test 2 vs test 4) and that the addition AC buff does not influence the effect of the CS (test 4 vs test 5).
The only explaination I can come up with for this 1st perception is that with increased weapon def, big stacks of weapon def on 1 item have increased effect due to the fact that weapon def is percentual and not accumulative (e.g 50 spr def (CS) > 50 spr def (20 boots, 20 gaunts, neck)).
The reason for the 2nd point is easy: like I said before, AC only has an influence on r-dmg and multipliers of that (the 300% dmg part of the Howling Sword skill) but will have no effect on the addition damage (the 200 dmg part of the Howling Sword skill). Thats why the 200 dmg will remail and what's left of the 300% dmg will reduce with additional AC.
On a side note: these results are only significant for a certain variety of skills: skills with a percentage of damage and additional damage (most BP and warrior skills). The rate in which each shield is more effective depends on the percentual dmg/additional dmg ratio. The more perentage damage the skill has, the better Gab becomes (r-hit is basically a skill which does 100% dmg and 0 additional dmg, that's why Gab did better in this test) and the more additional damage a skill has the better CS becomes (Helis does 250% dmg with 400 additional. CS will proove to be extremely effective against battle priests whereas against Hell Blade (300% 350 additional) the Gab's effect will be relatively stronger (though eventually CS will still stop more dmg probably).
Finally I'd like to thank Gary and Hito for helping me with these tests.
If there are any question marks regarding this whole subject I'd be glad to (try) answer them.