Virtal Machine preferences?

[p.echols at comcast.net]

I am getting ready to set up a virtual machine to install Windows XP.  (Time to quite dual booting)  From my scan through the archives of this list, it appears that VMWare Server or VirtualBox are the options.  (I have not pursued references to other options as these two seem to be what is used most.)

The first question: there some reason to use one over the other?  Features or ease of use?

The second question: when I am not using Windows (which will only be rarely) I assume I can shut down the VM and recover the memory / cpu resources for Ubuntu.  Are there complications with doing that?  

Thanks for any thoughts

P.E.