According to the Mozilla Developer Network (shorthand properties, Mozilla Developer Network, https://developer.mozilla.org/en-US/docs/Web/CSS/Shorthand_properties, accessed November 2015), shorthand properties are:
"CSS properties that let you set the values of several other CSS properties simultaneously. Using a shorthand property, a Web developer can write more concise and often more readable style sheets, saving time and energy." – Mozilla Developer Network, 2015
Unless strictly necessary, we should never be using longhand rules. When possible, shorthand rules are always the preferred option. Besides the obvious advantage of saving precious bytes, shorthand rules also help increase your style sheet's maintainability. For example, border: 20px dotted #FFF is equivalent to three separate rules:
border-style: dotted; border-width: 20px; border-color: #FFF;