Synthetic Lifelines have many advantages over steel lifelines:
Easy to inspect
Never cut your hand
Easy to install yourself
Dyneema is the material of choice for synthetic lifelines. Dyneema is soft on the hands, easy to splice, and stronger than steel with just a fraction of the weight.
Dyneema is a modern fiber made of High Modulus Polyethylene (HMPE) which offers incredible strength for its weight and size. They offer a strength greater than steel without any of the corrosion issues.
Synthetic lifelines offer a clear advantage in inspect-ability over coated steel lifelines. Steel lifelines live hidden in a plastic casing, slowly rusting away without giving any signs. When I replaced my steel lifelines, the cable was actually broken in several places inside the plastic casing. I only learned of this when I was coiling the old lifeline to recycle it. For two years, it had escaped my inspecting eye! When dyneema begins to wear, it will show clear signals in the form of chafe and fuzzing. There are also easy ways to rate how much residual strength is present based on the visual condition of the dyneema.
This link outlines how to inspect and assess dyneema
Steel lifelines don't have to live in a plastic covering making them impossible to inspect. The plastic covering can be stripped away leaving the wires exposed to the elements (which will accelerate their deterioration). As the steel wires weaken with time and use, they will begin to part. When you find the parted wire with your hand, you will learn why they call parted wires "Meat Hooks"! The wires tend to form little hooks on them as the wires break. Thanks to the ease of inspection, you are able to see that the lifeline needs replacing and have a very painful cut on your hand where the meat hook took a chunk out of you. Dyneema on the other hand offers the same ease of inspection as un-coated steel but while never forming meat hooks. The line is perfectly smooth to the touch and feels comfortable in your hand. I know I can hold onto the lifeline as I climb forward on the deck in rough weather without fear of injuring my hand.
The best part of synthetic lifelines is the ease of installation. All you need to do is make a few easy to complete splices. Simply run the line off the spool through all the stanchions until you reach the point where it will attach. Make an eye splice with a thimble and tie a lashing to the attachment point. Then cut the other end off the spool and make the same eye splice. I prefer the Mobius Brummel Eye Splice because it is very secure and will not slip.
Once the ends are eye spliced, you can use the mechanical advantage of the lashings to tighten the lifelines. The lifelines don't have to be cut to perfect lengths because you are not limited by the adjust-ability of a turnbuckle. If you are a little too close, you can simple move the splice further into the line. If you are too far, simply make the lashing a little longer. I personally prefer to make the lifelines a little short because they will stretch while settling in.
The gates are also very easy to make, simply splice the gate latch directly to the dyneema. The gate latch is perfectly happy connecting to a stainless steel thimble. I wove a lanyard onto the gate latches to make operation easier.
Synthetic lifelines are very forgiving to install. The process progresses quickly which can keep morale high during the installation. The most important thing to keep an eye on is when the lifeline passes through the shrouds. Usually the lifeline will pass inboard of the cap shroud and outboard of the lowers, but make sure there is a fair path for the lifeline before you put in the eye splice.
Overall, synthetic lifelines offer unmatched advantages over steel lifelines and truly are the superior material for lifelines on boats.