i like plex more because it recognized my media much more accurately than boxee, has an indicator for a played video, and also lets me manage cover art. the one thing that boxee does well is the apple remote controls (more intuitive), but then we also get less control over what we can do as far as cover art management.
although the social features are a nice add on, i wish there was a way to disable it for those of us that don't want to use that particular feature.