When it comes to virtualization on OS X there are three basic solutions: Parallels Desktop, VMware Fusion, and VirtualBox from Oracle. While VirtualBox is developed more as a free hobby and experiment package by Oracle, the other two are commercial packages that have been neck-and-neck in competition in the areas of speed, stability, and compatibility.
In its past few versions, Parallels Desktop has been significantly faster than VMware Fusion for a few tasks, which Parallels has regularly blogged about.
Today VMware released a new version of its Fusion virtualization software (available through its built-in updater) that in part offers a number of speed improvements that may reduce the performance lead that Parallels has developed.
For one, VMware is claiming that version 4.1 of its software can improve graphics by up to six times when running Internet Explorer and other programs like SolidWorks, especially when running on OS X 10.7.2. In addition, animations and graphics in full-screen mode should be smoother and snappier, and the loading time of Windows 7 virtual machines and transitions into and out of full screen are quicker.
Beyond speed improvements, the Fusion 4.1 update offers better support of NTFS volumes, and better integration with Apple's Full-Screen features in OS X Lion.
These improvements to the software are very welcome improvements for Fusion users, but time and testing will tell exactly how the new version compares with Parallels in performance.
Regardless of their differences, Parallels Desktop and VMware Fusion are both very robust virtualization options that can run many Windows software packages within OS X. VMware Fusion is offered for $49.99 (temporary promotional price) and Parallels Desktop is $79.99, though both have free trial options.