Both have advantages.
I am a die-hard fan of boxee and completely support its mission to create internet media content remote friendly. I just got windows 7 and happened upon WMC by accidentally pushing the button on my remote. After using both for a while here are my thoughts:
Internet Media: Although both contain applications for internet media, Boxee exceeds all expectations and leaves WMC everything to be desired. WMC is only linked to MSN online content which has podcasts from history channel and others, but hardly makes it worth checking for new content. Boxee, with its extensive online content and ever-increasing repositories, makes it unbeatable.
Local content cataloging: WMC uses standard titles for browsing and I found it fairly confusing at first to navigate. Boxee, although I found it annoying at first to use the system to recognize media, it has been incredible to have everything listed in that way and to have info about every episode. Again, Boxee surpasses WMC.
The only reason that I will still be using WMC is because of the soon promise of netflix. Microsoft has implied that they will be adding netflix onto their service. I believe that this will come sooner than it will to boxee. Although the look of WMC, the functionality of boxee wins every time.
System Commodore KERNAL/Commodore BASIC 2.0
CPU MOS Technology 6510 @ 1.02 MHz
Memory 64 KByte RAM
Graphics MOS Technology VIC-II (320 x 200, 16 colors, sprites, raster interrupt)