Here is a link to an exhaustive benchmarking test for Parallels vs VMWare. Good luck.
(Here's the target link: http://www.mactech.com/articles/mactech/Vol.25/25.04/VMBench... )
QEMU - http://www.nongnu.org/qemu/
VirtualBox - http://www.virtualbox.org/
Parallels - http://www.parallels.com/products/desktop/
VMWare Fusion - https://www.vmware.com/products/fusion/
Windows support ranges from good (QEMU, VirtualBox) to great (VMWare Fusion, Parallels). My personal pick is VMWare because of its excellent support for non-Windows OSes. I've managed to get Ubuntu, FreeBSD, DragonflyBSD, Haiku, and Plan9 working with it. Under Windows, I've been able to use a USB MobileConnect dongle and VPN, and if you happen to be running OS X Server you can even virtualize other OS X Server instances (Edit: I should mention that Parallels also has support for this).