Here is what to consider when buying a climbing sling or runner:
What Are You Using it For?
When checking out slings and contemplating that full-rack overhaul, the most important thing to consider is what exactly are you going to be doing with the slings you buy. A climber primarily ticking off those long alpine routes will be looking for a different sling than a climber using the slings for elaborate top rope anchors. If you need something super light, go for the super skinny slings. If you just need a workhorse that will work unfailingly for whatever application you may need, go for that slightly bulkier but ultra classic sewn nylon sling.
Generally speaking, the super skinny slings are going to be most aptly suited to the climber that takes note of the amount of weight they are lugging around. With the modern uber skinny slings you can shed weight and bulk making your rack lighter and far more compact. Drawbacks to the skinny slings are that they are, for the most part, extremely difficult to untie once a knot has been weighted. Nylon slings, and thicker dyneema slings will be much easier to untie, but won't be nearly as lightweight.
Dyneema vs. Nylon
All slings used to be made of nylon. Then a decade or so ago, slings started being made of Spectra, Dyneema or Dynex. These are super strong materials that are stronger than steel, don't wear out as fast and absorb less water than nylon. Because of their strength, they can be made in very narrow and light sizes, often half the weight of a nylon sling.
However, other than being cheaper, nylon slings have a big advantage: they absorb more force and act like shock-absorbers during a fall. Think of a dyneema sling more like a carabiner and nylon sling more like a climbing rope. When you load a nylon runner, it stretches and applies the force to you and your anchor or protection more gradually. This means that an anchor or protection piece may be more likely to hold. Check out this great video that describes the difference between nylon and Dyneema using drop tests. And here is a nice description of the pros and cons of Dyneema and Spectra versus nylon
So which is better? It all depends. If you are an alpine climber or climb where weight is an absolute premium, Dyneema is probably the way to go. If you are building and anchor or expect to fall a lot, nylon is the probably the way to go. We generally carry a mixture of both and adjust the mixture based on what type of climbing we are doing.
To sum it all up: when buying some slings, be honest about what you'll actually be using it for, and buy the sling best suited for that purpose. And be sure to check out our gear reviews to make sure you're getting the quality you desire.