What I like to do with all these poly's is take a look at their physical properties.The important one to me is the specific gravity. Less than 1 is going to float a fly real good, the greater away from 1 the better for nymphs and streamers.
Dunk some samples in a bowl of water and you will find that the physical properties show themselves right away, the heavier stuff sinks and the lighter stuff floats real good. You can find all you need to know about synthetic fibers by searching wikipedia using common or scientific names of the material you are looking at.
FYI, polypropylene's (Fly-Rite) selling point when introduced in the 70's was a specific gravity lighter than water.
These are some rules I follow when using synthetics. It was harder before internet to get this info, but it's all on the keyboard now. Have a good time searching.
"As far down the river as he could see, the trout were rising, making circles on the surface of the water, as though it were starting to rain."- E.H., The Big Two Hearted River