How do you decide whether to use a sling or a length of dynamic rope when you need a cow's tail or lanyard? The main advantage of a sling is that it is a normal piece of kit that you will always carry, so when it comes to clipping anchors in descent, or when rigging a top rope, you will usually have one available. Slings are strong, but Dyneema tape has very little elasticity and therefore doesn't like being shock-loaded, so this should be avoided if possible. This is normally easy to do with care, and if you don't have anything else available, always use a sling rather than nothing. A short length of dynamic rope makes the best cow's tail, but this requires an extra piece of kit that many climbers do not usually carry. I often use a home-made rope cow's tail when climbing, but also sometimes attach using a sling. As with all systems, it is important to realise the limitations of the equipment we use, and to remember that safety comes from an awareness of danger.
This is an excellent video from ENSA in Chamonix about different types of lanyards used in mountaineering, and demonstrating their advantages and disadvantages:
Lanyards for Climbing & Mountaineering
The photo below shows me with two clients, all attached using slings, and with the slings either under tension or with only a little slack in the system.