I am having both products and currently I prefer VMWare. One of the biggest reasons is, that all necessary drivers for Linux inside VMWare are part of the Linux distributions - e.g. CentOS and Fedora run without extra drivers installed.

Parallels brings their own drivers to install, which means not every Linux version is supported, the last time I tried it was lacking drivers for the newer kernels, this seems to have improved somewhat.

VMWare requires a few more clicks to configure with the upside that you have a clear configuration process so little magic is happening in the background - Parallels seems to automatically mount your drives in the VM which often is not what I want it to do.

Also, VMWare supports platforms beyond the Mac, so if you have a mixed environment, you can even move VMs between e.g. Windows and Mac machines.