this is still very debatable territory for me, regarding what is better.

A very pliable or very 'floppy' sling slings noticeably easier if you ask me
it simply doesn't  resist slinging as much. You can even rotate these slings
with barely any wrist motion.
using them feels so smooth
especially the somewhat thicker but floppy round traditional shepherd slings
I think they are worse at pouch orientation control with the wrist, but at the same time I can
sling with less wrist action which also produces less rotation of the pouch during rotation

On the other hand stiffness in sling cords resist rotation and therefor resist slinging
stiffness in a sling gets less with use, wear and tear
high stiffness requires a certain threshold of stone weight to open properly
but it can provide more direct control of the pouch angle.
However I am doubting whether making slings stiff is worth it at all
really thinking about it stiff sling just doesn't make sense

So I am growing more towards slings without reall stiffness and with round cords, they just feel smoother, more like how a sling is supposed to be
And if tangling/twisting starts being an issue, then simply make the sling thicker instead of making it stiffer (?)

I'd go with pliable, not so much for accuracy or control, but portability.

I can stuff a cotton string or mason line huaraca into my back pocket and not even feel it.

Who knows... maybe I'll change my tune when I get around to making a really stiff corn husk balearic.  Something sacrificed, something gained, it's all compromise and balance.

I feel like where I am with pliable vs stiff is;

If the cords are stiff enough to resist pouch rolling and dropping ammo from the sides of the pouch (only need reinforcement around the pouch for this) and twisting mid wind up and getting in the way of clean releases (so called: pouch angle control) *for your given ammo choice*, then you are in the clear.

I more or less subscribe to your stuff around stiff cords being effective at pouch angle control etc. My only qualm is that I feel like in reality you just don't need that much control and relatively pliable slings do just fine too for most ammo sizes/types. Perhaps the pouch angle is more stable when the cords themselves are under a lot of tension? As opposed to your free hanging twist test.

This is consistent with my personal approach of matching the sling to the ammo. Lighter and pliable slings for lighter and smaller ammo. Heavier and stiffer slings for heavy and large ammo.

I think this is a question of preference rather than what is better.  There are too many factors that go into a sling to claim that one characteristic is better than another (long vs short, full pouch vs split, etc.).  In some situations I think there can be a "better" choice to make, but there are many more variables that need to be accounted for in the decision.  Such as the projectile, style, distance to target, and not least of all the slinger.  Say we were going to have a competition, but it was required that everyone throws Figure-8 style.  I could be wrong, but I don't think anyone is going to grab a short sling with stiff cords for that.  And the reverse would be the case in CA's example on heavy projectiles and a short target distance.

That said, I vote for pliable. 

Pliable for me. I personally find stiff slings much less accurate and responsive.

I notice that when using a ~250 300 g stone and a really light sling, the efficiency is very high, there's no resistance to rotation
so it quickly feels as if you have to tune down power in order to gain control back. A stiffer / heavier sling doesnt rotate as freely and thus is already slowed down, as a result it feels as one can up the power. Reality of this  is I think a trick on the mind.
The heavy stone would leave the light sling just as fast or faster, and likely with less excessive spin, it's just because of how efficient  a such a sling combo + heavy stone is (sling no longer feels like its there) that one has to slow down in order to handle it.
In fairness this should be a good thing I think. For it should allow the user to sling in a more controlled manner
Of course if one wants to retain the feel of whanging a heavy stone, you could still shorten the thin sling or use the stiffer/heavy s less efficient sling.

J so it sounds like your views on slings has changed a bit. Is that fair to say? Do you over all prefer more pliable slings now?

I use thin cords  and really simple slings; Chumash and netted slings for the last month or two. I still use both grips, tight and wide depending on what I throw, where I throw and what type of spin I want.  I  also use rotational and no rotation styles but with rotations no more than 2 or 3.  But have dropped most heavy, thick slings from my arsenal instead finding other solutions. The only things I still see them really being good for is quickest reloading but the upsides of thick slings no longer weight up to the downsides for me. I find I generally sling without straining myself to the max and want the sling to do more of the work.
A light sling can sling light and heavy stones but I find I cannot sling light stones well in a heavy sling. A lot of my projectiles weigh as low as 70 grams