While I'm sure both github and gerrit have there technical merits and deficiencies, the overwhelming difference appears to be cultural and community, that are interesting to debate.
For the Jetty user base at least, there just appears to be more of a culture of contribution around github. The Pull Requests our project receives have continued to increase in frequency and quality, even since my last post on the subject. While it may have been technically possible for the same contributions to have been made via gerrit, it just didn't happen.
Regardless of technical merit, there is some zeitgeist around github and we are happy eclipse has a presence there so can get some of that!