What is the difference between using System.Windows.Automation and Microsoft.VisualStudio.TestTools.UITesting for WPF UI Test automation?

What is the difference between using System.Windows.Automation (commonly known as UIA) API and Microsoft.VisualStudio.TestTools.UITesting (one that is used in Coded UI Test behind the scene) for implementing UI Test automation of a WPF application programatically?

Is there any specific scenarios, restrictions or advantages/disadvantages?