I know coco needs to wet, not dry. That is irrelevant to the fiber content. They will both retain similar amounts of water, but coco that has too much pith drains too slowly. It's a practical concern. I have to wait on it to drain if I'm flushing or whatever, and I don't want to do that. If you don't care how slowly it drains, then it doesn't matter, but that's a big concern of mine. I don't have the patience for slow draining coco.
fibrous coco dries out much faster, which can only mean one thing, it's retaining less water. if it's draining faster than it would be fairly easy to assume that less water is being absorbed in the process, and the total level of saturation is less(if you add 4l to a plant and the one in the fibrous coco drains out 1l, and the more refined stuff only has .5l of runoff it's safe to say the medium has absorbed an extra .5l is it not?). speed of draining is the least of my concern, ideal conditions for my plants are much more important than a slight inconvenience/added wait time. I really don't care what type of coco that you prefer, use what works for you...I have been growing in nothing but coco for nearly 10 years, I'd say I've got a pretty good idea of what works best for me, and I prefer less fibrous coco, even canna is too fibrous for my liking. also perlite is completely pointless in coco.