Polypro floats and has no stretch...good for attaching a type IV throwable cushion or ski tow line.
Nylon stretches, resists chafing, doesn’t shrink when wet .......good for dock lines, anchoring or mooring. Nylon's ability to stretch makes it dangerous under strain. If the attached hardware comes loose the line may turn it into a projectile.
Anchor rodes and dock lines should be nylon. 3-strand has more stretch to it than braid, and if you leave the boat in the water during storms, 3-strand is recommended because it absorbs some of the slamming forces on the lines and hardware.
No line is chaf proof. Make sure you use chaffing protection of some kind if you slip the boat.