Polypropolene and Nylon Webbing

For the Same Width and Thickness, what is the differance between the two?

Polypropylene tends to be stiff, but it resists fatigue well and floats. It's the preferred plastic for things like waterski ropes and hinged tops on plastic containers. Incidentally, if you have yellow polypro line on your dive flag, I recommend changing that. I once had my flag come untied, and I didn't know it because the line floated.

Nylon is significantly more elastic, much stronger, abrasion resistant, and resists fading and UV deterioration in sunlight.

It seems to me like nylon webbing would be the better choice for a weightbelt or Hog harness.