Defense will do little to reduce the damage you take. Phalanx + Stoneskin is the way to go.
This really isn't true... classic example of taking a correct HNM mentality and trying to apply it to any situation.
The reason that the above argument exists is because HNMs have extremely high base damage (weapon damage). This negates the damage reduction from defense: no matter how low you get pDIF by stacking def, they will still do close to that extremely high amount of base damage.
Mandragoras and crabs that you'll use to skill your shield do not
have extremely high base weapon damage. Meaning that adjustment of pDIF by your additional defense from cocoon (or def food, equip etc) could actually make a difference.
To answer the original question then, look at the damage you take without cocoon or phalanx. If this amount is at all close to 19, then phalanx will definitely be your better bet. If it is a lot higher then that (I'd say 50+) cocoon might prevent more damage. Worth trying at any rate.
Unfortunately, HNM is the case where having a higher defense actually DOES more for you than otherwise. Your examples are almost exactly opposite.
What happens is that a pDif is generated: mobs attack divided by your level corrected defense. Cocoon is a great way to diminish pDif, but, as you've suggested, much of the damage generated by mobs (whether HNM or not) comes from their base damage, you can only decrease it so much.
Anecdotal case in point: I leveled PLD/BLU at level 51sh. We only gained one level, but Cocoon knocked a maximum of 2 points of damage off my average from when it was on to when it wasn't. Our defense is just naturally that much higher than the mobs' attack. Since most end game mobs have higher attack, defense has a little bit more of an input on what we're able to do.
Phalanx, on the other hand, will ALWAYS deduct a raw number off your damage taken. Depending on your enhancing skill, this number will quickly outpace the 50% defense bonus.
For most cases, Phalanx >>>> Cocoon. From HNM to XP to low level mobs (who you're probably flooring pDif on, anyway, but will still hit for some damage).