Whether using towing safety cables or towing chains, it doesn't really matter as long as you use one of them consistently and properly. Sometimes the term "safety cables" is used to refer to either one, but the primary difference between the two is pretty self-explanatory: Safety cables are durable cables, whereas safety chains are made of heavy-duty metal links.
The purpose of both pieces of towing equipment is to catch and support the load of the trailer if it gets disconnected from the tow vehicle. By crossing the chains or towing cables under the trailer coupler and attaching them to the tow vehicle with hooks, the safety cables act like a safety net in an emergency, catching the dislodged tongue.
While towing safety cables aren't designed to carry the fallen trailer across great distances, they do prevent it from crashing to the ground and spinning off into oncoming traffic. They give the driver enough time to pull over safely and figure out why the load came undone in the first place.
The most important thing to consider when picking out towing cables is the amount of weight you'll be carrying. That's because just like your tow vehicle and hitch, the safety cables are designed to carry a specific amount of weight. The capacity of the cables should exceed your gross trailer weight (GTW), the weight of the loaded trailer. Like trailer hitches, towing safety cables come in a variety of classes ranging from Class I (2,000 pounds (907 kilograms) GTW) to Class V (10,000 pounds (4,536 kilograms) GTW).
Another important thing to consider when setting up your safety cables or chains is the type of hook you want to use to connect them. S-hooks (metal hooks curved like the letter 'S') are a common option, but there are also slip hooks and quick links which close shut. Again, it doesn't really matter which you choose as long as you fasten them securely so they can't bounce loose, rendering your safety net useless. If you opt for the S-hooks, it's recommended that you also use rubber keepers to keep the hooks from bouncing out.
If your tow vehicle or trailer doesn't already have a place to connect the safety cables, you'll need to weld on a safety chain loop or bracket as a secure point of attachment. Again, make sure you pay attention to the equipment's class -- using only a Class II safety chain loop to tow your 5,000-pound (2,268-kilogram) boat won't do you much good.
After you've made sure the cables are securely attached to both frames, you'll also want to check to see that they've got a little Goldilocks action going on -- not too tight and not too loose, but just right. In this case, "just right" means not so tight as to prevent you from making a full turn but not so loose as to drag on the road. Too much slack in the chains could prevent them from doing their job.
So while it wouldn't be terribly catastrophic if some of you happy-go-lucky people were to drop your toast butter side down once in a while or occasionally catch a red light, towing is not an area where you want to be on the losing end. Towing safety cables and safety chains are part of a smart backup plan, whether they're mandated in your state or not.