There is a long list of differences. I'm sure you can find a lot of information on both products and maybe try MediaMonkey for yourself. The free version of MM offers everything that most users will need.
The main difference is that MM is good for organizing your music while XBMC offers almost no functionality in that department. If you are using VoxCommando it is important that your music be well organized with proper meta data. Even if you use XBMC to listen to music, I think it makes sense to have a program like MM for organizing it. It also has tools for ripping, burning, downloading album art etc. You have better control of your audio outputs etc. Basically you can probably expect that a program which is dedicated to managing and listening to music is going to be better (for music) than something like XBMC. MM 4 offers support for videos and other stuff too but I don't use it for that.
Some other key differences: MM will not stress your CPU the same way that XBMC (on Windows) will. So for some systems if you are really just listening to music and you don't intend to watch milkdrop visualizations or artist slideshows etc. MM makes more sense because it will probably use less electricity, and/or leave your cpu free for other thigns. MM also has slightly better integration with VoxCommando I think. It is a bit easier to do music requests and playlist manipulation etc, although XBMC is pretty good, especially if you are willing to play around with their JSON API a bit on your own using xJson.Raw actions.
One advantage of XBMC is that the API is all based on network communication so VC and XBMC don't need to be running on the same system. MM needs to be running on the same system as VoxCommando in order for VC to talk to it.
