I have purchased a bunch of different cords and rope with the purpose of experimenting and observing (possibly quantifying) the difference between these when braided as fibre or otherwise, with and without water soaking. Given the amount of slings I've made, mostly of the same sisal cord and hardware store jute, this is something I should have done quite a while ago. I mean, I've nearly done through 1.5km of sisal at this point in two 750m massive balls. The sisal is good stuff but needs a lot of hassle to maximise it's quality, jute I've always liked working with, but the durability does not impress me.
So, I've purchased-
-Linen cord 1.5mm (40m) I've actually used this before once, its very soft and lovely. I've nearly finished a rockman with this which I'll post when I'm done.
-Sisal rope 6mm (1m) Because generating fibre bundles from cord is time consuming, fiddly and requires a lot of pruning for high quality. I want to try rope.
-Hemp rope 6mm (20m) This is actually the first time I've worked with hemp, despite it's popularity and good rep on here. It's a massive pain to find here for some reason, it seems 99% of ebay and amazon listings are jute falsely advertised as hemp.
-Manila/abaca rope ~15mm (1m) Never seen or used this before, it's very tough stuff, makes sisal even seem a bit soft...
-I've also got some Hempex on the way which is synthetic hemp rope, hopefully it has the feel of natural cord but can withstand whipcracks. It's made from polypropylene.
-Not a natural cord for sure but I also bought some kevlar for whip crack reinforcement which I'm integrating into a few slings

My only concern so far is that it is quite slippery which might be where the hempex comes in.
Observations after soaking and initial work:
-Linen is so great to work with, I'm using fibre but it hasn't been soaked (95% sure it's all natural anyway). It's so damn soft it's hard to believe it's durable. It even smells lovely!
-Getting smooth lovely sisal fibre from the rope is a lot easier than the cord I've been using, this is definitely a new standard for me providing the fibre is equally good.
-Manila feels tough as nails by default, but after a soak it separates so easily into flexible thick fibres, I'm very optimistic about this stuff for sling braiding. It's super fast to produce fibres and they look real good stuff.
-The hemp and linen don't take as well to water, I'm pretty sure both are all natural in any case but they don't so much split into fibres as fibre bundles. The hemp smells pretty crappy but I think that's normal.
All in all I can't wait to get some braided samples made up and compared. The best cord I make currently is sisal fibre soaked/pruned, dried totally, braided, soaked again then dried... this takes a while.
The process I think I'll follow is non-soaked fibre vs soaked fibre braids, then I can soak each of the completed braids after to see if it makes a difference (probably soak just half for comparison sake). I'll then report on how flexible the resultant braids are and try to have a guess at wear resistance
Here are some pictures of the first steps: