Both Parallels Desktop and VMware Fusion deliver on the core goal of allowing Macintosh users to run Windows applications without needing to reboot their computers.
The biggest difference between them right now is that Parallels Inc.'s product is finished while VMware Inc.'s Fusion is still in beta testing. In fact, Parallels recently announced the availability of the third beta version of the next release of its software, which includes support for upgrading a virtual Windows XP system to Windows Vista, among other features.
The fact that Fusion is still in beta is rather evident. Several configuration dialogs include the sentence, "This device will be editable in a future release," and there are some minor stability problems.
Performance is another major difference, and it is linked to the fact that Fusion is still in beta. Although the time it takes to install or boot Windows or to run most applications is similar between both Parallels and Fusion, some actions that involve redrawing the screen are slower in Fusion, sometimes resulting in a sluggish feel by comparison.
Running benchmark tests within similarly configured virtual machines under each application reveals dramatic differences in some processor and graphics functions. (Parallels' scores for graphics performance are almost double those of Fusion.) The most likely cause for these differences is explained in a Fusion alert dialog that informs users that it is running in a debug mode that reduces performance and that can't be disabled in the current release.
The finished version of Fusion will tell a fuller tale on the performance front.
Both Parallels Desktop and VMware Fusion are easy to install. Both offer setup assistants that guide users through creating a virtual machine and offer configuration suggestions based on the operating system being installed and the hardware specifications of the Mac.
Fusion takes advantage of the dual-core processors used in almost all Intel Macs by allowing you designate whether the virtual machine will be able to use one or both cores. This is a nice feature because you can opt to retain the processing power of one core exclusively for Mac OS X and Mac applications, or you can throw all the processing power to the virtual machine.
Parallels makes setup much easier than Fusion with its "Express Windows OS Installation Mode" that performs the entire install process of either Windows XP or Vista for you. All you need to do is enter your name and Windows activation key. This is both extremely user-friendly and a great-time saving convenience.
Both products offer a set of tools and specialized drivers for Windows to enhance its performance in a virtual environment. These tools offer things such as a shared folder that can be accessed from Windows and Mac OS X, the ability to respond to the cursor appropriately as you move into or out of the window containing the virtual machine, and enhanced hardware support. Both products also offer a tool for compressing the hard drive image file that serves as the virtual machine's hard drive.
The boot process for virtual machines created with either Parallels or Fusion is essentially the same as the boot process for an actual PC. One difference is that Fusion provides you with a virtual BIOS configuration tool that can be used to change boot options such as the order in which the virtual machine searches for a bootable drive. For its part, Parallels manages these features through a virtual machine configuration dialog. Fusion also supports network boot from a Windows PXE server, which Parallels does not.
Although Parallels boots consistently, Fusion is prone to the occasional unexplained Windows boot failure (generally resolved by simply rebooting the virtual machine).