Both VMware Fusion and Parallels Desktop for Mac are excellent products, and both allow you to run many OSes quite well (including OS X now). In the end, when you decide which product to use, you should take into account what's most important to you.
Windows 7 and 8 are such a pleasurable experience that unless there's some driving reason otherwise, you should use either of those rather than XP under either virtualization product.
When it comes to whether you should use multiple processors or 64-bit virtual machines, it depends on your use. If you have a real need for either, and can articulate a reason for it, then use them. They do work well. With that said, if you don't have a specific need, then don't bother with multiple virtual CPUs. As for 64-bit, you should use it (especially in Windows 7 and 8) unless you have a driving reason not to.
Many people have the feeling of "more is better," but when it comes to RAM in the virtual machine, that is not necessarily the case. More RAM means longer virtual machine launch times, suspends and resumes. For most users, 1 GB of virtual machine RAM will work best for Windows 7 and 8. Use more than that only if you really know you need it. Gaming may do best with 1.5-2 GB of RAM if you can spare it.
In the vast majority of our overall tests, Parallels Desktop 8 won. Again, if you count up the general tests (including the top 3D graphics scores), Parallels won 56% of the tests by 10% or more. If you include all the tests where Parallels was at least 5% faster, as well as the balance of the 3DMark06 graphics tests, Parallels increased the lead further.
If you focus exclusively on 3D graphics, as measured by 3DMark06, Parallels won by an even larger margin. Specifically, Parallels won 62.6% of the tests by 10% or more, and was also a bit faster on an additional 8.9% more of the tests, and tied on the rest. In other words, Parallels Desktop 8 was noticeably faster than VMware Fusion 5.0.2 in 3D graphics.
If you are a traveler, Parallels Desktop has power management features that stretch your battery life. On the MacBook Pro, we saw about 40% more battery time on an idling virtual machine (a couple of hours of additional use in Windows 7) than we did on VMware Fusion in the same test.
To be clear, this article is not a product review; it's a benchmarking analysis. You should use it as part of your decision combined with other factors, such as product features, user interface, which OS you want to run, graphics capabilities and support to make your product choice.
One thing is clear: virtualization for the Mac works well. Really well. Even with virtualization running as well as it does, I expect that we'll see virtualization products keep getting better and better.
About the author(s)...
Neil is the Editor-in-Chief and Publisher of MacTech Magazine. Neil has been in the Mac industry since 1985, has developed software, written documentation, and been heading up the magazine since 1992. When Neil does a benchmark article, he likes to test the features that people will use in real-life scenario and then write about that experience from the user point of view. Drop him a line at firstname.lastname@example.org.