Standard concrete mixes are available. While cheaper, the drying times are significantly longer; a week versus overnight, for example.
Quick Cure Concrete
Quick curing concrete mixes come prepackaged and you only add water. "Quikrete," for example, comes in a 50-pound bag and takes only 24 hours to cure.
The general consensus is that you use the concrete that best suits your project. In the case of building a fence, you'll likely want to continue adding the support and rails as soon as the posts are set. In that case, the quick curing concrete will allow you to complete your project sooner.