When you are purchasing a crossbow sling the main factors that you want to take into account are durability, ease of movement/ weight, and comfort. Additional features you might want to look at are focused on the aesthetics, because this will be both an extension, and a compliment to your bow.
The durability is essential, because this will literally be your “right-hand man.” (Or left-hand for the lefties) Look for something that has a nylon shoulder strap as these tend to be resistant to moisture and mildew that the seasons bring. These nylon straps luckily do not compromise on the comfort, so they are a great option. I’m partial to the options that have swivels because they allow a greater range of motion, and movement tends to be effortless. Since you are going to be using this to carry around your rig, you are going to want something that is the perfect balance of lightweight and durability. Lightweight so that it doesn’t weight you down much and durable so that it doesn’t break on you the first time it is used. The aesthetics on the bag is the icing on the cake. One of the most popular designs today is a camouflage print. This ensures you won’t be spotted by your potential kill.