Plex is very good, and works well as a media server and DLNA server (I use it with two Roku 2XS's). There are also Plex clients for SmartTV's, NAS's etc, so it is flexible, but, MB3 is shaping up to, finally, be something that could actually replace WMC, and provide a heck of a lot more features and functionality on top. To be honest WMC for me is just about live/recorded TV now. All my media playback is via MB2, but I'm testing MB3 and really like what the devs are doing. I think one thing they need to do though, is lock down the features and start killing bugs to get the 'official' full release out. I guess because so many people are stepping up to help, the features keep piling in, pushing the full release further and further back. Where's MBT for example? This is going to be a killer app, but it's taking a long time to appear.
If MB3 really does tick all the boxes, and is family friendly and SLICK, then it get's my vote to replace WMC. I know the juices are flowing with the MB devs, but if they haven't got a full release out by mid-year, people might start losing interest.