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 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.