Just wanted to give my opinion after trying both. I'll edit as I get to know better.
-Mediaportal has gapless audio playback; VMC doesn't and that rules out VMC for music lovers.
-Mediaportal works out of the box with standard file formats that VMC is tricky to get to work properly with such as ogg (could be codec writers' fault not MS but MS should still fix it).
-Mediaportal has a more flexible interface for browsing. (By folders or tags while VMP is by tags only.)
Winner: Mediaportal by a long way
-Organization is better in VMC unless you a complex folder structure (in which case I can't compare the two)
-Photos are sometimes misoriented in Mediaportal (doesn't respect orientation tags?). That's easily fixable with a couple of remote control clicks. VMC has a filter that oversaturates images. That's much less easy to fix as you would have to identify the filter, invert it, and apply it to a duplicate copy of your library.
DXVA is important as it reduces processor demands, allowing smaller, cheaper, quieter, less power-consuming systems, and increases responsiveness (fast-forwards, seeking). Subtitles are important unless you speak all major languages. The only player that is free and uses DXVA while displaying subtitles is MPC-HC, which is also a top notch player in many other respects.
-You can use MPC-HC with the my movies plugin in VMC as a hack but it doesn't work seamlessly. MPC-HC as an external player works with no problems from mediaportal and takes no time to set up.
-Setting up DXVA without subtitles in VMC works more seamlessly but takes a lot of fiddling with codecs.
-VMC's user interface looks better and effectively displays information visually.
-User interaction is very well designed in VMP, simple, intuitive. Mediaportal has an unnecessarily complex interface and if you end up in the wrong place you can sometimes get lost and sometimes crash the system.
Winner: VMC by a long way
Ease of setup:
-For basic use VMC is simpler. Not many people in this forum will be in this category.
-To do anything beyond the basics (play mkv, change video player, adjust browsing method, etc.) mediaplayer is simple to set up while VMP is either difficult (look at the codec problems people have here) if it is even possible.
-Setting up control (e.g. with MCE remote) is simpler in VMC.
Winner: Tie (really, mediaportal is very easy)
Blu Ray/DVD: untested