I also tried the demo of Just Cause 2, which runs in Steam. Although it installed in both virtualization apps, I could only run it in Parallels, as the game requires DirectX 10. (Fusion only supports DirectX 9.0c.) Not only did it run, but it ran quite well. While Parallels support for DirectX 10 is marked as experimental, it worked well in my testing.
Both Fusion and Parallels have added support for Retina displays when running Windows: You can run at pixel-doubled or full Retina resolutions. Both programs also now support USB 3; Parallels includes Bluetooth sharing.
Parallels lets you drag-and-drop email attachments directly to Outlook in Windows, and includes a presentation wizard to help those using Windows to give presentations on projectors. You can also now drag and drop files and folders between Windows and OS X.
Virtualizing OS X and Linux
Want to test out some questionable piece of software? Use a virtual machine. Want to experiment with changing system settings? Use a virtual machine. Running a virtual copy of OS X can be handy for any number of reasons. But if you can accept some limitations, both virtualization apps run OS X very nicely.
Both of them make it simple to virtualize Mountain Lion, using either the installer file itself or the Mountain Lion recovery partition. Unfortunately, both suffer from the same shortcomings: You'll find that an OS X virtual machine has much less power than the Windows equivalent. You can't accelerate 3D graphics. You can't drag and drop files between the virtual OS and the real OS. You can't run a multi-display full-screen virtual OS X.
Parallels does one thing when running OS X that Fusion does not: You can copy graphics to and from the virtual OS X installation (Fusion will only handle text on the clipboard).
If you have an urge to get your toes wet with Linux, both Parallels and Fusion make it easy to do so. Parallels makes it a touch easier, though, as you can install Ubuntu (not to mention Chrome, Windows 8 preview, and Android) directly from the Parallels itself; Fusion requires you to find and download the disk images yourself.
Fusion now supports OpenGL acceleration in Linux, matching what you can do in Parallels. I tried some simple OpenGL games in Ubuntu 12.0.4, and they ran reasonably well in both. (I was unable to find an Linux-friendly OpenGL benchmark that would run in both virtualization apps.)
Integration with Mountain Lion
Both programs allow you to run Windows apps in full-screen mode, but they do so in different ways. That's because there are actually two ways apps can run "full-screen". In the first, the program's window simply expands to fill up all of the visible screen space.