Polyester and nylon rope are the two most popular types of rope for consumers today. Both offer significant advantages over natural-fiber ropes: they are stronger, more durable, and often cheaper as well. But the two are not identical; there are a few important differences that make polyester better suited for some jobs and nylon better for others.
Nylon is marginally stronger than polyester under steady pressure. The two are the strongest of all commonly available fibers, which explains their popularity. Nylon's small advantage over polyester expands significantly, however, when the stress is not steady, but applied in pulses. For shock jobs, nylon is significantly stronger.
The reason nylon is so much more effective for jobs with pulses and/or shocks is that nylon is more flexible. Nylon can stretch when greater force is applied suddenly, and then return to its original length. This "memory" feature of nylon makes it especially advantageous, since the stretching it can absorb does not change the tensile strength of the rope.
Neither polyester nor nylon is optimal for use in water, since both types of rope sink. But polypropylene rope is the only type of rope that floats, and the extra strength that polyester and nylon offer is worth the trade-off for most users. That is not to say the two are identical in terms of water. Nylon absorbs water, while polyester does not. This gives the advantage to polyester, which can be used for boats and pools without becoming waterlogged, while nylon takes a long time to dry out and is heavier and unwieldy in the interim.
Both types are highly resistant to sunlight, vastly preferable to natural fibers in rope, but polyester has a slight advantage. Nylon will sustain sunlight damage at a slightly higher rate, although it takes huge amounts of time for such damage to add up to a structurally significant change. Both are extremely resistant to other types of exposure, such as chemicals.
Polyester has another advantage over nylon in terms of abrasion. Nylon is more likely to sustain small snags during the course of use and wear thin at stress points.