- it is comfortable to grab whereas many other quickdraw slings are narrow and harder to grab when working a route
- they use rubber piece they call a "String" that does a great job of keeping the carabiner on the bottom of the quickdraw oriented correctly
- the String rubber piece is very durable. While it eventually will break, it last much longer that most other quick draw rubber parts. And, once it does break, you can buy replacement called the Petzl String.
Most people just go with the sling that their quickdraw comes with. But we have taken other carabiners we like such as the Wild Country Helium and built our own quickdraws using the Express Sling. If you buy a Petzl quickdraw like the Petzl Spirit Express, it comes with the Express Sling.The only downside to this runner is it is kinda expensive and is a little bulky. If you are an alpine climber or shaving every ounce, you don't need these. But if you sport climb a lot, these are the way to go.