If you spend a lot of time working just with Windows then both programs will allow your Windows virtual machine to expand into full-screen mode. The Windows desktop will fill the entire screen and hide the Mac desktop altogether, so that it looks and feels just like using an ordinary Windows PC - although you can still switch back to the Mac desktop at any time just by using the standard Command-Tab keyboard shortcut.
But most people will probably prefer to switch back and forth between Mac apps and Windows apps, so a better solution is to hide the Windows desktop altogether and simply display individual Windows apps floating on the Mac desktop so that they look just like ordinary Mac apps.
Unity And Coherence in Parallels Desktop or VMWare Fusion
Fusion calls this option 'Unity', while in Parallels it's called 'Coherence'. There are some minor differences between the ways that each program handles these modes, but they work in essentially the same way. You can copy and paste information between Mac and Windows apps, which makes it easy to switch back and forth between different apps, and you can tell your virtual machine to share Mac folders, such as your Home or Pictures folders, so that your important files are available to Windows apps too.
Hiding the Windows desktop also means hiding many important features, such as the Windows Start Menu and the Taskbar, so Parallels and Fusion both add an ingenious pull-down menu to the main toolbar that runs across the top of the Mac screen. These pull-down menus allow you to view many key Windows commands and controls right on the Mac desktop - which is something that still amazes me even though I've been using both programs for years. You can even put individual Windows apps into the Dock, allowing you to launch Windows apps stored on a virtual machine just as you would launch an ordinary Mac app.
Parallels does have a slight edge here, though. When running Parallels, you can use the Mac's QuickLook feature to get an instant preview of files that are stored on your Windows virtual machine. The latest version of Parallels also came up with a headline-grabbing trick by allowing Cortana - the Windows 10 rival for Apple's Siri - to run on Macs even when your virtual machines are hidden or running in the background. So you can just say 'Hey, Cortana' to issue voice commands to Windows 10 even while you're working in ordinary Mac apps. It's a neat trick - especially since Apple itself hasn't managed to get Siri running on the Mac yet - and the slick presentation of Parallels Desktop may have slightly more appeal for many users, even if the two programs are very similar under the surface.
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