VMWare! Three reasons:
1) WMWare is a stable, large, well established player (undisputed leader as far as I know) in the virtual machine software industry. To me this means that they have more resources, more experience, etc. etc. which in the end will result in a better product.
2) My experience: I thought it would be cool to move VMs between Mac host computers and Windows host computers, as I use both. Parallels support on the Windows side SUCKED. They hadn't updated the Windows version for about 9 months at the time I was trying this. I wasted hours trying to get my VMs to work under the Windows version of Parallels. VMWare has excellent software for both platforms.
3) Snapshots. VMWare Fusion v2 now supports the same (similar) snapshot technology that their Windows Workstation product has, very nice.