The Differences Between Metalness and Specular Workflows
You may have noticed that the newer metal materials now come in two workflow options; specular and metalness. The reason for this is so that our users have options for importing these textures into the diverse variety of 3d applications available. The choice of what textures to use in any given application can become very complex and daunting, however, here is a quick breakdown of the two options:
This is the industry standard which the majority of high-end 3d renderers utilize. Several examples include; Vray, Corona, Renderman, Arnold etc.
This workflow type is more appropriate for real-time PBR renderers such as the Unreal Engine, Unity, Quixel etc. However as the PBR workflow is becoming more and more popular, a number of traditional renderers are now adopting this workflow so that you can choose which you'd like to use. Usually this is represented by a "metalness" slider in the shader which lets you effectively switch between these workflows.
Here is a compatibility chart to see which applications support either workflow:
As you can see, the specular workflow works in all of these applications, whereas only a few support the metalness workflow. So stick to the specular workflow if you are in doubt.
For even more information on these two different workflows, be sure to check out CG Cookie's superb video which describes their differences in detail: