I've used both in the past. I lean towards VMWare Player, as I am a VMWare admin at work..so it's consistency among the products that sells it for me. With that said, I've found both products to work and function well and not cause significant issues.
The "one" thing that annoys me with virtualbox is the following scenario. I like to setup and test many things, but don't like to constantly load the OS when I fire up a new VM. For this reason, I prefer to install Windows, and then run "sysprep" and then have that hard drive file copied to an images directory. Then, when I fire up a new VM, I copy in the sysprepped Windows image hard drive, attach to it, boot the machine and windows is up and running in just a few seconds. Unfortunately, virtualbox makes you put the hard drives into the virtual media manager and when you try to fire up a VM using the same HD image file, virtualbox complains that the UUID number is the same. Now, I can go out with a command line tool and manually change the UUID number, but this is simply another step that isn't required under VMWare Player.
If you really want to take advantage of virtual machines, and can afford to have a second machine on your home network, you might want to consider looking at building a whitebox ESXi install. http://ultimatewhitebox.com/forum/showThread/460
. I've got a few of these at work, and they are fabulous with managing resources and such when you need to run a small LAN to test a handful of things together at the same time. The ESXi install itself is free.