This isn't really about Gerrit vs. GitHub. To be clear, we're mainly doing this for CI (i.e. Travis).
That said, we (the iOS team) plan for our workflow to play to GitHub's strengths—which also happen to be our personal preferences. In short, this means "amending patches" becomes "pushing another commit onto a branch." We've run into issues w/ rebasing & amending patches destroying our diff in Gerrit, and problems with multiple people collaborating on the same patch. We think GitHub will not only provide integrations for free CI, but, as an added bonus, also resolve some of the workflow deficiencies that we've personally encountered with Gerrit.