After using both extensively, for me I prefer XBMC. I found it less intuitive to set up but significantly more flexible and customizable once done so.
Among the features of XBMC that I enjoy over the WMC/MB setup is the ability to create a single database for all of my HTPCs and then pause/stop in one room and resume in another.
Additionally, the iOS integration and Airplay make the systems far more entertaining to use.
I also enjoy how much cheaper XBMC is to set up (lower hw requirements, no Win 7 license, etc).
Lastly, I find the XBMC community larger and more responsive so I believe help is easier to come by and updates are pushed out with greater ease.
There are a few things I miss with WMC/MB that I don't believe will ever be fully implemented in XBMC.
I'm curious what changes the WMC/MB community will have in store once Win 8 is released.
Ultimately, it will be user preference of course. I encourage anyone interested to give both an honest try and evaluate based on your needs and desires.